I truly cannot understand what most of America's Republican lawmakers are thinking.

I don't mean their manic drive to cut the number of Americans with health insurance by 24 million — or even the House Freedom Caucus' insistence on gutting regulations that require insurance to cover specific services and procedures (like pregnancy, mental health, and hospitalization). Republicans stake out these positions because of ideology, truly believing that such changes will represent an improvement (the explanation has something to do with increasing "choice"). I think that ideology is profoundly foolish, but I can at least understand how someone could come to believe in it.

I don't mean the unprecedented partisanship that led Mitch McConnell to deny hearings and a vote for Merrick Garland, President Obama's nominee to succeed Antonin Scalia on the Supreme Court. I consider such brass-knuckle partisanship deeply pernicious and civically corrosive. But I can at least understand how the enormous stakes in the nomination (potentially shifting the ideological makeup of the court several clicks to the left for a generation to come) led McConnell and other Republicans to conclude that they should use every conceivable means at their disposal to prevent Garland's confirmation.

Here is what I can't understand: FBI Director James Comey testified on Monday that Donald Trump's presidential campaign is under investigation by the FBI over its potential ties to Russia. Let's be clear about what this might mean: treason. We don't yet know what the outcome of the investigation will be (though subsequent press reports have certainly underlined the importance of seeing it through to the end). But the very possibility that a sitting president and his circle could end up credibly accused of having advanced the interests of a hostile foreign power and of having colluded with that power in an effort to undermine the campaign of the president's political opponent should be more than enough to persuade Republican officeholders and pundits to treat the investigation with utmost seriousness — and to distance themselves from the man at the center of the investigation until such time as he is cleared of any wrongdoing.

The most rudimentary instinct for political self-preservation — not to mention American patriotism — would seem to point toward the need to keep Trump at arm's length. And yet, that's not what Republicans are doing — at least beyond the few who have long viewed the president with suspicion. Instead, we had Rep. Trey Gowdy (R-S.C.) attempting to turn Monday's House Intelligence Committee hearing into an occasion to rail against leaks surrounding the FBI's investigation. This was then followed by the committee's chairman Rep. Devin Nunes (R-Calif.) running over to the White House on Wednesday afternoon to share what he clearly took to be evidence at least partially vindicating the president's otherwise unsubstantiated assertion that he had been wiretapped by President Obama — an act that would seem to undermine the credibility of the committee's own independent investigation of the Trump campaign's ties to Russia.

Leading Republicans appear to be willing and even eager to tie their fates to Trump.

We've been through this kind of dynamic before, with Richard Nixon and the Watergate scandal. Plenty of Republicans rallied around him through months of bad stories that implicated senior members of the Nixon administration and only reluctantly abandoned the president when it became impossible to deny he was guilty of numerous crimes.

But there are important differences between Nixon and Trump. For one thing, Nixon was a lifelong Republican with strong allies throughout the party. For another, Nixon had just won re-election in a historic landslide, giving him an enormous supply of goodwill in the GOP. Neither is true of Trump. And then there's the fact that Watergate was the first scandal of its kind, making it difficult for rank-and-file Republicans to predict just how bad it would become. That is obviously no longer true in our post-Watergate era.

Finally, there's the relative gravity of the allegations in the two scandals. The Watergate break-in itself was obviously a crime, but what led to Nixon's downfall was the cover-up, which implicated the president in multiple acts of obstruction of justice, abuse of power, and contempt of Congress. That would have been more than enough to impeach Nixon, remove him from office, and indict him. Bad? You bet. But far from treason.

The allegations swirling around the Trump campaign are far more serious.

The issue is not, as some Trump defenders have been claiming, that Trump wants to change American policy toward Russia, making the bilateral relationship less adversarial. George W. Bush and Barack Obama made similar efforts, and even if the attempt seems comparatively foolhardy in the wake of Russia's annexation of Crimea and related meddling in Ukraine, there's nothing illegitimate about a presidential candidate proposing such a shift in foreign policy.

But what would be illegitimate, and perhaps even treasonous, is an effort to soften American policy toward Russia's actions in its near abroad without a clearly stated policy rationale and in return for help from Russian intelligence in defeating a domestic political opponent. "Promise to help us in Ukraine and we'll help you win against Hillary Clinton by releasing stolen emails that make her look bad": That and other possible acts of collusion between Russia and the Trump campaign are what the FBI is investigating.

Once again, we don't know what the investigation will uncover. Maybe there was no such collusion; maybe there were no treasonous acts. We can't yet say. Until the investigation is complete, it's seems both smart and fair-minded to withhold final judgment.

But until then, it also seems crucially important to withhold explicit support from the man at the center of suspicion. Political good sense as well as personal integrity demand nothing less. Why can't Republicans see that?