(CNN) Evan McMullin, the independent challenging Utah Republican Sen. Mike Lee, said in their only debate Monday night that Lee's actions around the 2020 election and the January 6, 2021, attack on the US Capitol were "a betrayal of the American republic."
Lee, meanwhile, said he accepted that President Joe Biden in 2020 had won the presidency in the "only election that matters -- the election held by the Electoral College." The senator defended his actions that day, pointing to his votes to certify states' Electoral College results.
Their clash came on the day elections officials in Utah began mailing ballots to voters.
McMullin describes himself as conservative but has said he would caucus with neither party if he defeats Lee. He is attempting to unite a coalition of Democrats, independents and anti-Donald Trump Republicans -- and he got an assist this spring when Utah Democrats opted to endorse him rather than field their own candidate. But in Utah, even that coalition might not be enough. Trump won 58% of the vote there in 2020.
McMullin's entrance into politics came in an effort to serve as an antidote to Trump. He ran for president as an independent against Trump in 2016. He drew 22% of the vote in Utah, well behind Trump's 46% and Hillary Clinton's 27%. Among those who voted for McMullin in 2016 was Lee, who said at the time that it "was a protest vote."
Here are four takeaways from their Monday night debate:
McMullin's sharpest attacks on Lee came after a moderator raised the topic of the outcome of the 2020 presidential election.
"You were there to stand up for our Constitution. But when the barbarians were at the gate, you were happy to let them in," McMullin said.
Lee pointed out that ultimately, he accepted the Electoral College vote.
"Yes, there were people who behaved very badly on that day. I was not one of them. I was one of the people who tried to dismantle that situation," Lee said.
McMullin, meanwhile, said Lee only voted to accept states' electoral votes after no other plan to keep Trump in office materialized.
"You voted to certify the election in the last moment," McMullin said. "In the same way that someone knows that a plot that's not quite working out ought to abandon it, that's what you did."
McMullin repeatedly cited text messages reported by CNN in April between Lee and Trump's then-White House chief of staff Mark Meadows in which the two communicated about efforts to overturn Biden's victory for weeks.
In early December 2020, Lee began texting Meadows about the idea that states could submit alternate slates of pro-Trump electors to Congress on January 6. Lee ultimately voted to certify states' electoral votes.
McMullin said Lee was working "to keep a president who had been voted out of office, according to the will of the people, in power despite the will of the people."
He pointed to Lee's November 7, 2020, texts to Meadows asking him to help Sidney Powell -- one of the most prominent attorneys fronting lawsuits that supported Trump and made accusations of widespread election fraud -- get access to Trump.
He mocked the pocket Constitution that Lee carries, telling the senator that it is "not a prop for you to wave about and then when it's convenient for your pursuit of power, to abandon without a thought. That's what you've done with that."
Lee shot back: "I disagree with everything my opponent just said, including the words 'but,' 'and' and 'the.' An information-free, truth-free statement -- that's something of a record."
"There is absolutely nothing to the idea that I ever would have supported or ever did support a fake electors plot," Lee said. "Nothing. Not a scintilla of evidence suggesting that. Yet you continue to suggest that with a cavalier, reckless disregard for the truth."
In an effort to cast Lee as extreme, McMullin invoked Utah's other GOP senator: Mitt Romney.
Criticizing Lee's approach to fiscal measures, McMullin said he "routinely votes against bills that would improve water infrastructure."
"Meanwhile, Senator Romney has worked hard and consistently over the last three years," McMullin said. "He works with Republicans and Democrats, Senator Lee, to deliver for Utah. And he voted in favor of the bipartisan infrastructure bill that you voted against. And now tens of millions of dollars have already been directed to Utah to improve our water infrastructure."
Lee responded: "Yeah, I voted against that bill -- a bill that spent well over a trillion dollars more than we have on all sorts of things that weren't appropriately federal."
Romney has stayed out of the race.
Lee, in an appearance last week on Fox, made a plea directed at Romney: "Please get on board. Help me win reelection," he said. The move seemed designed less to win over Romney than to rile up Lee's conservative base.
Trump followed Lee's pleas to Romney with a statement in which he called McMullin "McMuffin" and said that Lee was being "abused, in an unprecedented way" by Romney.
Lee said he was "thrilled" with the Supreme Court's decision to strike down the landmark 1973 Roe v. Wade decision that had made abortion legal nationwide. He said he believes states should decide how to regulate abortion.
"This is where it should remain, because it's within the states that we can achieve the most consensus and protect the most babies," Lee said.
McMullin, meanwhile, sought to find a middle ground on abortion rights, saying that he opposes "abortion on demand" but also opposes state legislation to force young rape victims to carry their pregnancies to term.
"Some of these bills that I see being passed around the country are extreme," McMullin said.