(CNN) Even in his waning days in power, President Donald Trump is still finding ways to inflict his signature loyalty tests on the Republican Party.
As the President continues to deny his election loss, Republican members and aides on Capitol Hill are watching with angst as Trump threatens to unravel their entire year-end legislative agenda. After Congress left town this week, Trump unexpectedly posted a video screed eviscerating the hard-fought, bipartisan coronavirus package that passed the Senate 92-6. And Trump on Wednesday followed through with a promised veto of the bipartisan National Defense Authorization Act, a defense policy bill that's essential for governing the future of the Pentagon.
Through all that, Trump is obsessively pursuing any tactic that could overturn the results of the election, encouraging an already-doomed effort by a small group of House Republicans to challenge the Electoral College results in Congress on January 6 and lashing out at any Republican who calls into question the efforts' success -- even threatening to primary the Senate's No. 2 Republican over his opposition to the futile endeavor.
"He's coming unhinged," one GOP aide told CNN.
While Congress is already slated to return to Washington next week to potentially override Trump's veto of the defense authorization bill, Trump's latest criticism of the stimulus package and government spending bill could prove even thornier if he follows through on his criticism and vetoes that, too -- a move that at a minimum could delay any assistance from reaching Americans.
There is no predicting how many Republicans will continue to support a package that Trump has so publicly blasted. That has left aides on both sides of the aisle attempting to game out worst-case scenarios -- including a government shutdown that could last through the end of Trump's time in office.
If Trump waits to veto the bill, it could leave little or no time for the current Congress to override it. If that happened, a newly sworn in Congress that includes a larger Republican minority would have to vote on the legislation all over again. And, while Congress passed a seven-day continuing resolution to ensure the government did not shut down while the larger bill was being processed, that funding runs out Monday at midnight.
House Republican lawmakers held a private call on Wednesday afternoon to discuss next steps in the face of Trump's threats to scuttle the coronavirus relief package. House Republican Leader Kevin McCarthy said that Trump hasn't explicitly committed to vetoing the stimulus and government funding legislation up to this point, according to a person on the call.
McCarthy ambiguously told House Republicans they needed to find a way to address the President's concerns, but that hasn't quelled the frustration from some members. Rep. Don Bacon, a Nebraska Republican, said Trump had thrown Republicans, who voted for the package in large numbers, "under the bus," with his panning of the bill Tuesday night, according to a person on the call.
"I stand by what I said," Bacon said in a statement later, noting the call was members-only and off the record. "The President's Secretary of Treasury helped negotiate this bill and encouraged us to support it. Furthermore, the President was updated on this bill every step of the way by GOP Leadership," he added.
Texas Rep. Kay Granger, the top Republican on the Appropriations panel, warned that Trump's decision to attack the bill had already damaged its prospects to a dangerous degree in terms of whether it would be signed into law.
At least one member on the call mentioned the possibility of Republicans going on TV to tout the positive elements of the bill -- a tacit acknowledgment that that is where Trump gets his information, the person said.
In an attempt to try to stop any more leaks from happening that revealed the party divided, McCarthy eventually cut off questions during the call, according to a person familiar with the call, a sign of the delicate balance GOP leaders are walking thanks to Trump's threat to the spending bill.
By January 6, Republicans could be put in an even tougher position if Trump's House allies gain a senator's support to object to the Electoral College results, forcing votes in both chambers on whether to reject a state's electoral votes. It's a politically toxic vote for Republicans, forcing them to choose between standing with Trump or honoring the will of the voters.
Behind the scenes, Majority Leader Mitch McConnell has actively discouraged any Senate Republicans from joining the effort. Senate Majority Whip John Thune publicly pushed back on efforts to challenge the results of the election during a Joint Session of Congress, saying it would "go down like a shot dog."
For McConnell and Thune, the matter of simply stating the obvious -- that Joe Biden is the President-elect in McConnell's case, that a floor effort to overturn the results would fail catastrophically in Thune's -- served as the ultimate betrayal to Trump.
Trump's assistant stunned GOP senators when an unsolicited e-mail showed up with a PowerPoint slide that showed McConnell's polling in his 2020 Senate contest. Trump appeared to believe it showed his endorsement of McConnell, and a subsequent robocall on his behalf, was crucial to McConnell's victory.
McConnell won his race by nearly 20 points.
In Thune's case, the President wasted little time taking his 280-character blowtorch to a popular member of the Republican conference considered a top candidate to take over as GOP leader when McConnell departs.
"Republicans in the Senate so quickly forget. Right now they would be down 8 seats without my backing them in the last Election. RINO John Thune, "Mitch's boy", should just let it play out. South Dakota doesn't like weakness. He will be primaried in 2022, political career over!!!"
Trump's browbeating of his fellow Republicans in Congress follows a familiar pattern that's occurred throughout his presidential term, where at various points a GOP Senator or House member would raise concerns, only to face Trump's social media wrath.
What followed, according to several lawmakers who experienced it, was an explosion back in their home states or districts. Offices were barraged with furious callers and primary challenges were explored by local up-and-comers who pledged unyielding fealty to Trump.
"It's like nothing I've ever seen," a GOP senator told CNN. "You get singed by it once and realize OK, there's no real upside to going in that direction again."
It's a reality that has infuriated Democrats, many of whom regularly talk of hearing privately from their GOP colleagues how unseemly they find Trump's behavior. "But always privately. Always," a Democratic senator told CNN. "What an embarrassment."
As such, Republicans have repeatedly fallen in line behind Trump, out of fear of a Twitter rebuke or throwing his weight behind a challenger that pushed some of Trump's early Republican critics out of office or into retirement.
But congressional Republicans might finally push back in Trump's waning days. The first test of how effective Trump's final loyalty demands are may come on Monday, when the House has scheduled a vote to override Trump's veto of the National Defense Authorization Act. In his veto message Wednesday, Trump charged that the bill was a "gift" to China and Russia, a criticism that's befuddled lawmakers and aides in both parties.
The annual defense policy bill has been signed into law every year for six decades. The legislation provides pay raises for service members and typically passes with a veto-proof bipartisan majority in both chambers, just like it did earlier this month.
Trump, in effect, is asking Republicans to stand with him and vote against the troops. He demanded that the bill include a repeal of a law that provides legal protections to social media companies, even though it's unrelated to the defense legislation. And he wants to remove a provision that would lead to the renaming of bases named for Confederate leaders.
The bill, known on Capitol Hill as the NDAA and named this year after retiring GOP Rep. Mac Thornberry of Texas, passed earlier this month 335-78, easily clearing a veto-proof two-thirds majority, with Republicans voting 140-40 in favor. But House Minority Leader Kevin McCarthy and Minority Whip Steve Scalise have said they will vote to sustain Trump's veto, as have the President's Senate allies Lindsey Graham of South Carolina and Josh Hawley of Missouri.
It's still unclear how many Republicans will join them. A group of House Republican Armed Services Committee leaders, including Thornberry, the panel's ranking member, issued a statement last week touting the cyber provisions in the bill following the massive data breach of US government and private companies, arguing the legislation provides "critical safeguards to protect the information and capabilities most foundational to our nation's security."
The statement did not mention Trump or his veto threat, however.
The 37 Democrats who voted against the bill may also switch sides to override the veto, meaning fewer Republicans would need to rebuke Trump for the veto override to be successful. If the House does override the veto, McConnell has taken steps so the Senate can hold the same vote, which may not occur until the hours before the new Congress is sworn in on January 3. The Senate passed the bill earlier this month 84-13.
The fight over the defense authorization bill is only a prelude to a larger battle in Trump's mind: Congress counting the Electoral College votes on January 6.
The outcome of the Electoral College tally is not in doubt: There's no way for Republicans to change the outcome with Democrats in control of the House. But Trump has turned to the vote as a last stand for his baseless and false claims the election was stolen from him, and he's got a willing band of House Republican allies leading the charge.
Trump met this week with the group of House Republicans who are planning to object to President-elect Biden's wins in six states. If they convince a senator to join them, the Republicans can force a vote in both chambers, something Senate Republicans leaders are trying to avoid.
Rep. Mo Brooks, the Alabama Republican leading the effort, has predicted Senate Republicans will join in the objection, though none has committed to doing so. When Sen.-elect Tommy Tuberville of Alabama signaled an openness to objecting, Trump called him up and publicly praised him.
It would be quite an opening act for Tuberville to cross McConnell, who is famous for keeping his Senate conference united, but it could be an early sign of how Trump will continue to have influence among his Republican backers even after he's out of the White House.
"Tommy Tuberville serving as the tea leaf for the future of the Republican Party was not on my bingo card," one senior GOP official with close ties to Senate leadership said with a laugh. "But seriously, he better have some good advisers on this. Based on the last month his current position is the last place I'd want my boss to be."