Sen. Susan Collins (R-Maine) (AP Photo/J. Scott Applewhite)

It is at least within the realm of possibility this election that a Republican senator could support the Democratic presidential nominee.

On Tuesday, one of the Senate's most moderate Republicans, Susan Collins of Maine, announced she would not be voting for Trump, outlining in a column in The Washington Post why she thinks he is "unfit" for the presidency. She didn't say who she'd be voting for instead, but in an interview with the New Yorker in June, she left the option open to vote for Hillary Clinton.

UPDATE: In an interview on CNN Tuesday, Collins said she won't vote for Clinton. "I may well end up writing in a name for president, something I've never done before."

At least one House GOP lawmaker, retiring Rep. Richard Hanna (N.Y.), said he'd vote for Clinton, and two dozen or so Republican figures from past and present have said the same.

Defections from the Senate are pretty rare in recent history. When parties were more fluid in the 19th century, this kind of thing happened all the time. But cross-party presidential endorsements have dropped off a cliff as parties have solidified -- nay, calcified -- into the two very separate factions they are today.

To that point, Collins reiterated it's unlikely she'd vote for Hillary Clinton. And we can see why; voters don't seem particularly enamored with the idea, at least when it comes to their own ticket-splitting. As our own Philip Bump explored recently, the phenomenon of voting for a senator from one party and a presidential candidate from the other is the lowest it's been in decades:


(Philip Bump / The Washington Post)

That partisanship has filtered up to the Senate, where nowadays supporting the other side's presidential candidate is often met with punishment, either from one's party or the voters, said Donald Ritchie, a former official historian for the U.S. Senate.

"It's very, very difficult for them to step outside their parties in a way that you could 40 or 50 years ago, when the Senate was much more independent," he said.

The Fix spoke to Ritchie and Robert David Johnson, a history professor at Brooklyn College, to learn more about those moments in American history where senators have defected during a presidential campaign -- and what happened to them because of it. Here are some notable cases, in chronological order.

1916: When you ask senators or historians to name some of the chamber's greatest members, Robert La Follette, a Republican from Wisconsin, inevitably comes up. He's credited with being a leading figure in the progressive movement (the parties were, of course, different back then). The 1916 presidential election was the lead-up to America's involvement in World War I, and these progressive Republican senators liked the other side's approach to try to stay out of it. La Follette led a small handful of senators in "effectively communicating they would not be unhappy with" Democrat Woodrow Wilson winning reelection, according to Ritchie. But Ritchie notes this wasn't a formal endorsement of Wilson -- just a nudge in his general direction.

1924: A few years later, La Follette ran for president, this time as a candidate of the Progressive Party, which he formed specifically to run. His entry made it a three-way race between him, President Calvin Coolidge and John Davis, a little-known *former Democratic member of Congress. For many progressive Republican senators and even some Democrats, La Follette's party became a kind of de facto Democratic Party challenge to Coolidge, since progressives on both sides of the aisle weren't fans of the more conservative Davis. Democratic Sen. Burton Wheeler of Montana ended up being La Follette's running mate on the Progressive Party ticket.

But the cross-party support from his colleagues wasn't enough: Coolidge's 25-point win margin in the popular vote was among the largest in presidential history. La Follette came in third.

1948: The prospect of war again divided some factions of a party. Sen. Glen Taylor, a colorful cowboy and Democrat from Idaho, was concerned his party's president, Harry Truman, was leading the nation to the brink of war with the Soviet Union. So he endorsed Henry Wallace, a peacetime candidate running under the Progressive Party. "I knew I would probably kill my chances of being re-elected in 1950 if I threw in with Henry," Taylor told a newspaper in 1948, according to his obituary in the New York Times.

He was right. Wallace ended up naming Taylor his vice presidential pick. They failed to win any state in the general election, and afterward, Taylor's state party tried to kick him out. Two years later, Taylor lost his reelection.

1960: If it's not war dividing a party, it's a safe bet that religion is. Catholic presidential candidates in particular have struggled to hang onto some of their party's members. In 1960, John F. Kennedy's nomination caused some Southern Democrats to defect. They ended up voting for their colleague, Sen. Harry Byrd from Virginia, a conservative leader in the Senate and notable opponent of school desegregation. Byrd was never on the ballot, but he got 15 electoral votes from states like Alabama, Mississippi and Oklahoma.

1964: Conservative Republican Barry Goldwater's 1964 Republican nomination caused a lot of heartache within the parties (and he remains the presidential hopeful most often compared to Donald Trump). Much of the unsettled feelings on the Democratic side stemmed from the Civil Rights Act, which President Lyndon B. Johnson signed into law a few days before the Republican presidential convention. Around that time, Sen. Strom Thurmond, a Democrat from South Carolina and one of the Senate's most vocal opponents of the law, defected from his party to support Goldwater.

And he never looked back. For the rest of his life, Thurmond was a Republican, and his switch was a key flashpoint in the North-South political realignment. Thurmond ended up being one of the longest-serving senators in U.S. history.

2004: Zell Miller, a conservative senator from Georgia, was one Democrat who didn't switch parties during that realignment (which is still going on to some degree today). But in 2004, he crossed over and supported President George W. Bush's reelection campaign, even speaking for Bush at the Republican convention. (Miller holds the distinction of being one of the few people to be keynote speakers at both party's conventions.) The two had been acquainted when they served as governors, and Miller said he respected Bush's commitment to tax cuts.

In a sense, Miller was already on his way out of the Democratic Party. He wasn't running for reelection, and before endorsing Bush he published a book: "A National Party No More: The Conscience of a Conservative Democrat," in which he criticized the party for being too liberal and elitist. He left the Senate shortly after Bush's reelection, joined a law firm and got a gig on Fox News.

2008: Joe Lieberman, a former senator from Connecticut, has lived several different political lives. He was originally elected as a Democrat, and he was Democrats' vice presidential nominee in 2000. But after losing his 2006 Senate primary, he switched to his own party -- "Connecticut for Lieberman" -- and won in the general election. He became an "independent Democrat," and showed that independent streak two years later when he endorsed Sen. John McCain (R-Ariz.) for president. As a result, he almost lost his powerful seat chairing the Senate's homeland security committee -- but Democrats decided to let him keep it. He retired from the Senate in 2013.

*We originally said Davis was a member of Congress when he ran for president. He had finished up his two terms nearly a decade earlier.