With one exception, Senate Republicans are standing by President Trump, who is facing allegations he pressured Ukraine to dig into his potential 2020 opponent.

They could regret that.

We don’t know what is in the whistleblower complaint centered on Trump and Ukraine, but since news of its existence and some of its contents started surfacing last week, it has consistently suggested the worst for Trump.

Consider: After reports that the complaint centered on a “promise” to a foreign leader, we learned it involved Ukraine, a country Trump has political ties to through his former campaign head Paul Manafort.

We then learned that Trump was said to have pressured newly elected Ukrainian President Volodymyr Zelensky to investigate the family of former vice president Joe Biden, whose son served on the board of a Ukrainian energy company. The Wall Street Journal reported Trump brought up Biden and his son eight times in the call with Zelensky.

After we learned all that, Trump didn’t deny he brought up Biden in the call, though he maintains he did nothing wrong. On Monday, he said he did not make military aid conditional on Ukraine investigating Biden and his family.

And now we learned through The Washington Post on Monday night that Trump stopped military aid to Ukraine (aid even Republicans in Congress wanted to go to the country) a week before he made the call and apparently talked about Biden.

Maybe there’s a legitimate explanation for all this. A senior administration official told The Post that holding up the military aid “had nothing to do with a quid pro quo” and that Trump was concerned about corruption in Ukraine.

Trump himself confirmed Tuesday that he withheld military aid from Ukraine, but he gave a different explanation: He said it was over his concerns that the United States was contributing more to Ukraine than European countries were.

Maybe what Trump did was unethical, but not outright illegal or impeachable. Splitting that difference is how Republicans stood by him after the release of the Mueller report, which found Trump’s campaign welcomed Russia’s help but didn’t actively collude with it to win the 2016 election.

On that thin line is where Sen. Marco Rubio (R-Fla.) balances. He told The Post’s congressional team Monday: “He simply raised the issue of Biden, and I don’t believe he should have done it. But that in and of itself is not an impeachable offense, as some people claim. Now, the second thing you raise, [a quid pro quo], he denies, and so do the Ukrainians. If alternative information emerges, we have a different set of circumstances, but that’s not before us right now.”

But other Senate Republicans are being less circumspect and doing their best to defend Trump when they don’t really know what he did.

Sen. John Cornyn (R-Tex.), who is up for reelection this year, cast aspersions on the whistleblower, whose allegations were deemed credible by a Trump-appointed watchdog in the intelligence agency. “What makes you think he is [a whistleblower]?” Cornyn asked, as The Post’s Felicia Sonmez, Mike DeBonis and Erica Werner reported.

Sen. Ben Sasse (R-Neb.), a former Trump critic who recently got Trump's endorsement in a primary, simply said: “I'm not doing any interviews right now.”

Others are trying to focus on what Biden may or may have not done, but the facts to allege wrongdoing just don’t seem to be there.

Then there were the straight-up defenders of Trump.

“I believe that President Trump is going to blow you away with his willingness to disclose and be transparent about this phone call,” said Sen. Lindsey O. Graham (R-S.C.) in a radio interview with Hugh Hewitt, “because I think he did nothing wrong, and he has nothing to hide."

Maybe Trump has nothing to hide. Maybe we already known the worst of the story, and at the very least, it’s spinnable for Republicans. (We wish he didn’t bring up his reelection on a diplomatic call, but the president should have the prerogative to talk to foreign leaders how he wants, or something like that.) That seems to be the pattern for most of Trump’s scandals. Senate Majority Leader Mitch McConnell (R-Ky.) even defended Trump after he tweeted that four minority members of Congress, all U.S. citizens, should “go back.”

But there’s also the possibility Trump did something indefensible, and that it eventually comes out. That’s the gamble most Senate Republicans are taking right now.