But it's getting harder and harder for Republicans to use that caveat, especially now that Comey has crossed the battle lines — and apparently has proof to back up that Trump acted improperly.

Now, Republicans face a difficult choice: Do they keep giving the president the benefit of the doubt that he's done nothing improper? Or do they take concrete steps to get more evidence, wherever it may lead?

The “evidence” option is scary for Republicans. To start seriously questioning the president's version of events, they will have to turn down a path that could lead to the last place they've ever wanted to go: Direct confrontation with their party's first president in eight years, and perhaps, ultimately, a direct challenge to his authority.

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Already, one of the most powerful Republican members of Congress, Rep. Jason Chaffetz (R-Utah), has started walking. The chair of the Oversight and Government Reform Committee is tasked with overseeing the government, and on Tuesday night, hours after the existence of the Comey memos was first revealed by the New York Times, Chaffetz requested the FBI hand over all its records of communication between Trump and Comey.

Right now, it's just a request. And it's not really much different in substance than what lots of Republicans have been saying over the past few days: They'd like Trump to provide information. They'd like the alleged “tapes” of his Oval Office conversations with Comey, or a transcript of his conversations with Russian diplomats, so they can get a clearer picture of what happened.

“We need to have all the facts, and it is appropriate for the House Oversight Committee to request this memo,” Ashlee Strong, a spokeswoman for House Speaker Paul D. Ryan (R-Wis.), said in a statement to The Post about Chaffetz's actions.

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But the White House (and, in the case of the Comey-Trump memos, the FBI) could just decide not to hand anything over.

Here's our fork in the road.

Chaffetz is prepared for resistance in the Trump administration. He issued a threat Tuesday that represents a significant escalation between Congress and Trump: a subpoena. If the FBI doesn't hand over its Comey-Trump communications, Chaffetz indicated he will issue a legal directive requiring them to, or else face fines, jail time or a vote of contempt of Congress.

“Truth,” Chaffetz told The Post's Carol Leonnig Tuesday night about what he's seeking. “Let's see how real these memos are.... And see where they take us.” (Worth noting: Chaffetz could be departing Congress at any minute, leaving this investigation in limbo.)

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But as Chaffetz hints, more information could come with more problems for Republicans.

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Trying to find out what happened is a direct challenge to the Trump administration's version of events. They deny Trump ever asked Comey to lay off an investigation into one of his top former advisers.

Let's say Chaffetz and the rest of the committee do get the Comey memos (with or without a subpoena) and find something close to a smoking gun — communication by Comey that directly contradicts the White House and suggests Trump tried to obstruct justice.

Then what?

Republicans would not be able to turn back from what would likely happen next.

Congress could hold hearings into whether Trump obstructed justice, at which point the battle lines would become Trump vs. an FBI director whom a significant number of Senate Republicans didn't think Trump should have fired.

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Congress could also vote to appoint an independent commission — made up of bipartisan political luminaries outside Congress — to produce a report on what happened. Their other option is to appoint a special prosecutor, usually a lawyer outside the government given wide latitude to investigate and, if necessary, charge people with crimes. So far, very few Republicans support either of those measures.

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After that, Congress's choices get even more antagonistic: They could vote to hold the president in contempt, or even start proceedings for the “i” word.

And it all starts with Republicans deciding that what the White House says about these damning news stories is worth questioning.

Many smart Republicans have no doubt played this scenario out already to its end conclusion: Once you start demanding more information from the Trump administration, it becomes nearly impossible to turn back from what they find.