Senate Republicans are digging their own political graves.

If Republicans do not allow at least a few witnesses into the Senate impeachment trial, they will pay a heavy political price if evidence emerges later proving deep corruption by President Trump.

Such evidence almost certainly will emerge.

When former national security adviser John Bolton goes on 60 Minutes showing contemporaneous, detailed notes of Trump outlining an explicit, thoroughly indefensible quid pro quo with Ukraine's president, Republican senators will be blamed for complicity.

When political fixer Lev Parnas releases yet more audiotapes, more photographs, and more emails all indicating Trump's perfidy, the Republican senators will be blamed.

When other witnesses turn over new evidence while looking for immunity in the investigation of Trump by the U.S. attorney for the Southern District of New York, the Republican senators will be blamed. They will be seen as having engaged in a cover-up. They will be seen as having undermined justice. They will be seen as having embraced all the dangerously extravagant theories expounded by Trump’s lawyers during the trial, such as Alan Dershowitz saying that if a president self-defines his own political interest as being equivalent to the national interest, he can do almost anything he wants.

In truth, Republicans’ electoral prospects this fall are already dire. Already, 27 Republican House members are retiring or seeking uncertain political promotions, many of them from competitive districts, whereas only nine Democrats are doing so. Six Republican-held Senate seats are thought to be in danger, whereas only one Democratic seat is seriously threatened. Democratic voters and donors are so fired up against Trump that House Republican Leader Kevin McCarthy is warning of a fundraising crisis, with Democrats “kicking our ass” on that metric.

Senate Republicans covering for Trump are letting all their chips ride on the intensity of the voters from Trump's base to carry them through to reelection, but that’s a risky bet. Especially with highly controversial candidates, those supporters can suddenly reach a breaking point where enough is enough. I’ve seen numerous elections where support for high-risk candidates suddenly evaporated, resulting in massive, sudden swings in the polls — one from a dead heat to a 25-point loss in just three weeks.

Republicans who don’t hedge their bets by at least allowing witnesses will have no chance to survive if Trump takes a dive. The remaining Republicans would face increasing odds of finding themselves a powerless minority against an enraged and emboldened Democratic majority absolutely out for blood.

Republican senators ought to allow witnesses because it’s the right thing to do — on multiple levels. Yet if they lack the character or wisdom to do the right thing for its own sake, then pure political fear ought to supply what James Madison called “the defect of better motives.” For political self-preservation, Republican senators should call witnesses — even if only to cover those backsides that McCarthy says are being kicked.