Political obsessives in Washington and around the country are waiting with bated breath to see whether Congress will "release the memo" that House Republicans say proves FBI impropriety in the Russia-related investigation of President Trump.

The FBI, for its part, has warned of "grave concerns" in releasing a memo with severe "omissions of fact." It will be up to President Trump whether to heed them.

But if the memo does see the light of day, let's be honest about who the real target audience is. It's just 51 people — the Republican members of the Senate. Trump's fate rests more in their hands than Special Counsel Robert Mueller's.

It seems more likely than not that Democrats will retake the House in November, with the building wave of GOP retirements perhaps the best indicator that Republicans themselves are expecting a rout. If Democrats win control of Congress, they will then have the simple House majority required to impeach Trump. What they almost certainly will not have, however, no matter how well the midterms go, is the two-thirds Senate majority needed to convict and remove him.

Given the difficult map this year, Democrats may not even be able to take control of the Senate. But for the sake of argument, let's say they win a 52-48 majority. They would still need 15 Republicans to vote to remove Trump from office.

Would Senate Republicans ever turn on Trump? That's a tall order. But contrary to the expectations of liberals who believe all elected Republicans indiscriminately enable Trump, it may not be an impossible one, at least compared to the last two presidents threatened with impeachment.

Remember the '90s? Even with 55 votes in the Senate, Republicans were never going to be able to complete their impeachment drive against President Bill Clinton because there were no Democratic senators willing to vote to remove him. Only five House Democrats voted for any of the Clinton articles of impeachment. The GOP was never going to get 12 senators.

Similarly, if party leaders had allowed antiwar Democrats to move forward with their efforts to impeach President George W. Bush, they might have won over Republicans like Ron Paul or Walter Jones in the House, but zero Senate Republicans could have been persuaded to convict Bush.

In Clinton's case, Democrats simply did not believe that the underlying offense that led to the president's perjury and alleged obstruction of justice — consensual sexual relations with White House intern Monica Lewinsky — was serious enough to justify removing him from office. And in Bush's case, Republicans would have regarded the president's impeachment as the criminalization of policy disagreements (particularly over the so-called war on terror), and they overwhelmingly still agreed with the Bush policy in question.

Things might be different with Trump.

If Mueller is able to present clear and compelling evidence of collusion between Russia and the Trump campaign, with the president's knowledge, Republican votes for impeachment really are gettable in a way that they were not in past cases. This situation would be fundamentally different than the impeachment talk around Clinton and Bush. And some Senate Republicans really might turn on the president.

Obviously, the threshold for GOP senators to turn on Trump would be much higher than for Democrats, for whom the Trump Tower meeting alone suffices. For Republicans, "collusion" would likely have to mean some direct involvement in stolen Democratic emails, cooperating with the creation and distribution of Russian fake news, or at bare minimum a well established awareness of and coordination with what the Russians were doing. They would have to see a clear quid pro quo.

For many GOP lawmakers, even that wouldn't be enough. Many have indeed shown that they'll back Trump no matter what. But it is not that hard to imagine a set of facts that would cause principled Russia hawks like Ben Sasse, Marco Rubio, and Lindsey Graham to turn against the president. And once a few dominos fall, it makes it easier for the others to topple in turn.

Remember, too, that the two biggest reasons that Republican lawmakers continue to back Trump is that they see him as a vehicle to accomplish their legislative goals, and they fear a backlash from the Trump-loving base were they to turn on him. But in a post-midterm world in which Republicans have been roundly shellacked, Trump's legislative agenda would be dead on arrival, and his base would be disillusioned. Republican senators might well see fewer reasons to stand by their man.

Of course, a lot would have to happen to get us to this point. What little we know of the Mueller probe suggests we are presently far from this outcome. None of the indictments handed down so far even attempt to establish the existence of a broader conspiracy between the Russians and the campaign, not even in the case of George Papadopoulos, who appears to have at least attempted collusion. Cooperators could be rewarded with sentencing leniency while pleading to things more significant than process crimes — maybe the fact that they weren't charged with worse offenses doesn't mean anything, maybe it does.

All the intrigue surrounding the Trump White House points to an obstruction investigation, centering around the firing of former FBI Director James Comey. That's where the memo, the anti-Trump FBI texts, and the complaints about former Deputy Director Andrew McCabe, whatever their merits, come into play: The more doubts Republicans have about the impartiality of the investigation, the more evidence they will demand of Mueller.

The degree to which Senate Republicans can be persuaded that the Justice Department, even under Trump, is sketchy and stonewalling also strengthens the president's hand. Firing Mueller, as he reportedly considered doing last year, is still a no-go zone that could turn Senate Republicans against him. So, for the moment, is firing Attorney General Jeff Sessions, though there is now a constituency for his removal among House conservatives.

The man to watch is Deputy Attorney General Rod Rosenstein, who is overseeing Mueller and would ultimately decide what to do with any report he produces. If he emerges as a villain among Republicans in the fight over the memo, it is valuable impeachment insurance for Trump. If Trump is instead goaded into overreaching against Rosenstein and Mueller, it could make his position more precarious. And it might just encourage some senators to pick principle over party.