Before Ronald Reagan became the face of the conservative movement and the closest thing imaginable to a Republican saint, he had been a New Deal Democrat who’d campaigned for Franklin D. Roosevelt.

But moving from the far left of the political spectrum to the far right – without stopping in the center – did not make Reagan a traitor, at least not in his mind.

“I didn’t leave the Democratic Party,” the Gipper would say. “The party left me.”

Fifty years later, it’s good to remember that parties are voluntary associations, and nobody has to belong to one. If you don’t like where the train is heading, you can always do what Reagan did and get off. You always have a choice.

So far, Maine Sen. Susan Collins has chosen to stay on board the Republican Party, even as it becomes more deeply enmeshed with the fortunes of Donald Trump, a president who gives off a stench of venality like bad breath. Having a Republican in the White House means Republicans can pass laws and confirm judges. The stalwarts may not like how Trump makes his money, but they need him where he is.

Ever since the 2016 election, Collins has tried to maintain a kind of independence where she sometimes criticizes Trump but generally supports him with her votes.

Finding a stable place in the middle has never been harder and she may soon be forced to make a choice – reject Trump’s brand of Republicanism, or claim it – and all that comes with it – as her own.

There have been some hints lately that point to which direction she may be headed.

In the weeks before the 2016 election, Collins published an op-ed in The Washington Post saying that Donald Trump was “unworthy of being our president” and would not get her vote.

“Some will say that as a Republican I have an obligation to support my party’s nominee. I have thought long and hard about that, for being a Republican is part of what defines me as a person,” Collins wrote. “It is because of Mr. Trump’s inability and unwillingness to honor that legacy that I am unable to support his candidacy.”

But asked a few weeks ago if she would support Trump for re-election, Collins had warmed considerably: “Truly, I think it’s too early to make a decision,” she told New York Times Magazine reporter Mark Leibovich. He followed up, asking why she remained a Republican.

“I haven’t given it a lot of thought, to tell you the truth,” she said.

That may not be an endorsement, but moving from “he’s unworthy to serve” to “eh, maybe” is quite a leap. And so is the distance between saying that Republican Party values “define me as a person,” to saying that they don’t mean enough to deserve much thought.

What has Trump done in the last two years to earn that?

His business, his phony “foundation,” his campaign, his transition – even his inauguration parties – are all the subject of investigations that have resulted in convictions or pleas to multiple felonies by his closest associates. If he were not president, he could have been indicted for campaign finance violations, according to court documents.

But in an interview on CNN last weekend, Collins dismissed the mounting evidence of criminality saying “clearly, this was not a good week for the president” and suggested his problem had been that his campaign team had been “inexperienced.”

As the investigations heat up, Trump is going to need more of this kind of help, not less. He holds nearly 90 percent approval ratings among Republican voters, and as long as that’s true, it’s unrealistic to think that the party establishment will dump him. They are going to need people like Collins to lend him their credibility, even as Trump holds on to his base by lashing out against immigrants or sexual assault survivors or by making reckless, crowd-pleasing moves, like last week’s announcement of an abrupt withdrawal of troops from Syria.

Tennessee Sen. Lamar Alexander, a pragmatic Republican who served with Collins in an era when across-the-aisle cooperation wasn’t anathema to Republican voters, announced last week that he would not run for re-election in two years. He joins Sens. Bob Corker and Jeff Flake, who both retired this year rather than face Trump loyalists in primaries at home.

Retirement is an option for Collins, but it’s not the only one.

She could cut loose from Trump by resigning from his version of the Republican Party and stay in the Senate as an independent. She could quote Reagan and say, “I’m not leaving the party, the party left me.”

That is a choice. Moving closer to Trump is also a choice. There may not be any others.