Impeachment trial: The six Republican senators Trump and McConnell should worry about Historic Trump impeachment calls for a new chapter in 'Profiles in Courage.' From Alexander and Collins to Murkowski and Romney, 6 possible authors.

Ira Shapiro | Opinion contributor

Show Caption Hide Caption Why a president can be impeached and remain in office Impeaching a U.S. president might not be the be-all-end-all for their career. We explain why this is the case.

Since the 2018 elections produced a Democratic majority in the House, every discussion about impeachment has begun and ended with the assumption that the Republican-controlled Senate would brush aside any impeachment, line up behind Majority Leader Mitch McConnell and exonerate President Donald Trump. As Brookings scholar William Galston told The Washington Post last fall, “There's a reason why 'Profiles in Courage' is a very short book.”

This historic moment would be a good time for a new chapter. If we look at individual senators, men and women facing the most consequential decision of their careers, it seems quite possible that Trump’s presidency does not rest on a strong foundation; rather, it hangs by a slim thread.

Just four Republican senators will make the difference between a quick, cursory trial and calling witnesses who could turn the tide toward conviction and removal. Here are six GOP senators who might do Trump in:

►Mitt Romney of Utah. The former Republican presidential nominee called Trump’s pressure on Ukrainian President Volodymyr Zelensky “wrong and appalling.” This reaction was predictable from Romney, who spoke out against Trump in 2016 and has continued to be critical. In an October interview with USA TODAY, Romney, 72, described himself as free to do "entirely what I believe is absolutely right. ... And I hope that my kids down the road will say, 'Yeah, you know our dad, our grandfather, our great-grandfather was a person of integrity and honored his oath of office.' ” That sounds like a man who is ready to draw a sharp contrast with Trump.

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Maine's independent tradition

►Susan Collins of Maine. Collins disappointed many supporters and friends, including me, when she voted to confirm Brett Kavanaugh to the Supreme Court, and she faces a very tough reelection campaign this year as a result. But Collins has proved her independence in the past. She angered McConnell by backing President Barack Obama’s economic stimulus in 2009 and opposing repeal of the Affordable Care Act in 2017. Collins came to Washington in 1974 to intern for then-Rep. William Cohen, a member of the House Judiciary Committee, which was then considering President Richard Nixon's impeachment. She also played an active role in the impeachment trial of President Bill Clinton in 1999. Collins cherishes her position in Maine's long line of independent senators — above all, the iconic Margaret Chase Smith, whose statement of conscience attacking Sen. Joseph McCarthy came more than four years before the Senate and the country turned against him.

►Lisa Murkowski of Alaska. Murkowski’s reaction to Trump’s treatment of Zelensky was instantaneous: “You don’t hold up foreign aid that we had previously appropriated for a political initiative. Period.” She has expressed discomfort with McConnell’s outspoken commitment to coordinate trial strategy with Trump. Murkowski waged an unlikely write-in campaign to win reelection to the Senate in 2010 after losing to a Tea Party extremist in a primary. She stood alone among Republicans to oppose the Kavanaugh nomination, and she joined Collins and Sen. John McCain of Arizona to block repealing the ACA. This headline from Alaska Public Media captures her fierce independence: “Murkowski, true to form, breaks with GOP colleagues on ethical questions about Trump.”

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Virtually every list of potential Republican defectors starts with Romney, Collins and Murkowski. Here are another three who should keep Trump and McConnell up at night:

►Lamar Alexander of Tennessee. Perhaps the only person in American history who has been a governor, Cabinet member, presidential candidate and senator, Alexander is probably the most respected Republican senator. Alexander expressed dismay about the Senate’ s hyperpartisanship back in 2011 by resigning from McConnell’s leadership team in order to engage in more bipartisan legislating. Now, at 79 and retiring after this year, Alexander doesn’t want to rile the Trump base in Tennessee. His highest priority may be to ensure that his friend, Bill Hagerty, succeeds him in the Senate. But will that really be enough to align him with Tennessee’s other senator, Marsha Blackburn, an extreme conservative who is one of Trump’s most ferocious defenders? Alexander began his storied career as a staffer for the great Howard Baker in 1967. He should be fully capable of finishing it with the type of courage and independence that Baker showed during Watergate and the Panama Canal treaty debate.

An agonizing choice for McSally

►Richard Burr of North Carolina. As chairman of the Senate Intelligence Committee, Burr has earned respect for working with Democrat Mark Warner to steer the committee’s scrupulously bipartisan investigation of Russian interference in the 2016 presidential election. Burr has announced that he won’t seek another term in 2022. He might be reluctant to break with North Carolina’s other senator, Thom Tillis, who is up for reelection and is all in with Trump. But few people know better than Burr how Trump’s pressure on Ukraine to investigate the Bidens detracts from the real steps needed to protect our national security and the integrity of our next election.

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►Martha McSally of Arizona. Perhaps no one faces a more agonizing decision than McSally, appointed to the Senate to fill the late John McCain’s seat shortly after she lost an election for Arizona’s other Senate seat. McSally served 22 years in the Air Force, rising to the rank of colonel. She was the first American woman to fly combat missions and the first woman commander of a fighter squadron. McSally also sued the Defense Department in 2002 over its policy requiring women to wear full body covering when going off-base in Saudi Arabia. Last year, she gave stunning testimony that she had been raped by a superior officer. Given her patriotism, courage and outspoken commitment to women’s rights, McSally may have less in common with Trump than any living person. Will she once again put "country first," or decide she has already shown enough courage to take a pass this time?

When the chips are down, these six Republican senators and others could prove to be knee-jerk defenders of the president, no different than Reps. Devin Nunes and Jim Jordan. But if Trump is banking on that, he could be underestimating senatorial pride.

Ira Shapiro, a former Senate staffer and Clinton administration trade official, is the author of "Broken: Can the Senate Save Itself and the Country?" and "The Last Great Senate: Courage and Statesmanship in Times of Crisis." Follow him on Twitter: @ShapiroGlobal