(CNN) Defense Secretary James Mattis is learning to jump in the political ring on Capitol Hill.
The retired four-star Marine general was confirmed as President Donald Trump's Pentagon chief to great fanfare among Republicans, but many congressional defense hawks were disappointed with his opening act as he was outmaneuvered by others within the administration and his defense budget fell short of fulfilling their desire to inject the military with a massive spending boost.
Now there's been an "evolution" in recent weeks, lawmakers and congressional aides say, Mattis has gotten more involved with the politics of Capitol Hill and helped Republicans sell their desired defense budget increases. CNN interviewed more than a dozen lawmakers, congressional aides, defense officials and lobbyists about Mattis' relations with Congress, most of whom spoke on condition of anonymity to offer a candid assessment of the defense secretary's performance.
They say Mattis' performance in a gauntlet of congressional hearings -- four committees in four days -- was impressive, as he gave a blunt assessment of the military's readiness problems and endorsed the higher budget topline congressional Republicans are pushing, even though it differed from his own budget request.
They also gave him high marks for getting involved in the House's debate of the National Defense Authorization Act, writing a letter to support one amendment and calling a lawmaker personally to urge her to drop a controversial provision.
"I think we're finding him to be more and more interactive with Congress now that I think he's got a better grip on this," a Republican House member told CNN.
But Mattis, who has often sought to stay above the political fray, still struggles with the politics of the Trump administration and a chaotic White House. The disconnect was clear last week when Trump rolled out a ban on transgender service members over Twitter — catching the Joint Chiefs off guard and directly contradicting Mattis' desire to study the issue further before making any decisions.
Republican lawmakers also still have lingering concerns that Mattis is not doing enough to solidify his standing within the administration, particularly when it comes to budget matters. They worry he will continue to lose political battles to Trump's budget director, Mick Mulvaney, a former congressman who has frequently clashed with Republican defense hawks over the military budget.
"He's a military leader, he wins wars, he inspires people. He's been fighting for us. But he's not an experienced infighter on the budget issue," said a Republican senator.
Mattis could find a new White House ally in retired Marine Gen. John Kelly, who was sworn in as Trump's chief of staff on Monday. But at the same time, Mattis has clashed with Trump's national security adviser H.R. McMaster as Trump's national security team has struggled to come up with a strategy for the war in Afghanistan that the President will endorse, according to congressional and administration officials.
Mattis, who retired from the military in 2013 as chief of US Central Command, was cheered by Republicans for his military bona fides and his blunt, straight-talking style. He was confirmed as defense secretary on the first day of the Trump administration. But key deputy positions at the Pentagon remained vacant for months.
There were several false starts with the administration's picks for Army secretary and Navy secretary withdrawing, and several senior positions were unfilled over disagreements between the Pentagon and White House. Mattis faced resistance from both the White House and Republicans over his favorite for policy chief, Anne Patterson, and an official has yet to be named to the role.
Republicans also grew frustrated that Mattis kept President Barack Obama's Deputy Defense Secretary Bob Work in the role until a replacement was confirmed, which only happened earlier this month.
"His background is not in politics -- that's why he needs good people around him," said a Republican House member. "You have an Obama holdover as deputy and his people and all these acting (officials). ... You just go down the list. And it's a problem."
House Armed Services Chairman Mac Thornberry publicly blamed Work and other Obama holdovers in acting roles for the Pentagon's $603 billion budget proposal. The Trump administration touted a 10% increase above the Budget Control Act Spending caps, but Republicans charged the Trump budget was just a 3% bump above Obama's plan and well short of the $640 billion they were seeking for the military.
Navy Capt. Jeff Davis, a Pentagon spokesman, acknowledged there had been some challenges with personnel early on, but he said that Mattis was "non-denominational" when it came to the political leanings of the people he wants working for him. Mattis is concerned with picking the best candidate for the job, Davis said.
On the congressional concerns about the budget, Davis noted that Mattis considers "fiscal solvency" to be part of national security.
Congressional aides say Republicans were also concerned when Mattis instructed the services to quit talking about details of the military's readiness problems, which Republicans have focused on as a key reason increased spending is needed.
But those early worries largely dissipated when Mattis testified before the Hill, where the Pentagon chief said he was "shocked" by poor military readiness and he endorsed extra spending.
"If this is the will of the Congress, I'm confident that the commander in chief would be in your corner all the way," Mattis said.
One Republican congressional aide noted that Mattis took the rare step of actually endorsing the military services' $30 billion "wish lists" of items that the Pentagon didn't include in its budget request.
Mattis' standing was also bolstered when the defense policy bill was on the floor earlier this month.
He helped Ohio Rep. Mike Turner, a senior Republican Armed Services member, with a rare letter weighing in on a specific amendment. Turner was trying to kill the creation of a new Space Corps, but the House Rules Committee did not allow his measure to get a vote.
Then Mattis conducted some behind-the-scenes lobbying, making a phone call to Missouri Republican Rep. Vicky Hartzler to ask her to withdraw her controversial amendment targeting transgender health costs for service members.
The amendment, which was defeated in a 209-214 vote, threatened to overshadow the typically bipartisan defense bill, and many Republican leaders preferred to avoid the fight, according to congressional aides.
"There's been an evolution," one House GOP aide said of Mattis. "At the end of the day, it's a two-party political system. It's not the same as being a CoCom (combatant) commander."
But Mattis' efforts on the amendment eventually came back to bite him, with Hartzler going around the Pentagon straight to the White House. She lobbied for the White House to kill the Pentagon funding for medical treatments.
Mattis had urged her to wait for him to study the issue, and he told Congress he was taking six months to make a decision. Instead, Trump blasted out over Twitter he was instituting a full ban on transgender service members, catching the Pentagon off guard and unaware of what the President meant.
While Republicans say Mattis has improved on the Hill, they're still concerned he will lose out to Mulvaney again when next year's budget is released.
Mattis has vowed that next year's budget will fund a rebuilding of the military after he's had time to conduct a full strategy assessment, but there are already warnings that the Office of Management and Budget (OMB) is looking at lower numbers than congressional defense hawks will deem sufficient.
"The Trump administration really hasn't given him the deference that he needs to move OMB," said another Republican House member. "The fact that he is actively participating in voicing support or opposition for provisions in the NDAA shows increased activism. Now he just has to do it in his own administration."
Lawmakers and aides say Mattis has preferred to stay out of the political fights inside the administration, an instinct they say stems from the military's apolitical ideal. But if he wants results, Republicans argue, he'll have to be willing to step in the ring to slug it out.
"If he wants a 5% budget increase annually, if he wants to escape (continuing resolutions) CRs and omnibuses, then he cannot do it by strictly being apolitical," said a Republican congressional aide. "If he decides that's what he wants, he can get it. So far, he's chosen not to go out and get it, which I think confounds people."