Republicans in Congress could break ranks with President Trump and support impeachment if evidence emerges that he directly linked Ukraine military aid to an agreement by Kyiv to investigate political rival Joe Biden.

Congressional Republicans are skeptical Trump told Ukraine President Volodymyr Zelensky that assistance from Washington was contingent upon his government launching a probe of Biden’s dealing with Ukraine while he was serving as vice president. But Republican sources told the Washington Examiner Tuesday, just before House Speaker Nancy Pelosi announced an impeachment inquiry, that enlisting a foreign capital to target a Democratic presidential contender would constitute an abuse of power.

A Republican insider who has been privy to conversations on Capitol Hill said, “If there is evidence of a quid pro quo, many think the dam will start to break on our side.”

“Maybe if he withheld aid and there was a direct quid pro quo,” said a chief of staff for a House Republican.

But Republicans in the House and Senate are speculating that the facts will fail to prove that the president committed impeachable offenses, just as it did in the case of alleged collusion with Russia.

Trump acknowledged telling Zelensky that Biden ought to be investigated, and the White House did delay the military aid appropriated for Ukraine by Congress, although it has since been delivered. But most Republicans suspect Trump held up the assistance over his usual complaints that the United States is footing too much of the bill and American allies should contribute more.

That is why they are eager for Trump to release the transcript of his telephone call with Zelensky, as he has since said he will do. Still, many Republicans, especially in the Senate, are treading carefully. In addition to the transcript, they want to review the unusual whistleblower report that brought the allegations to light, and any other related information, before clearing Trump.

“I want to make sure I know exactly what happened before I issue some kind of judgment,” Tennessee Sen. Lamar Alexander said. “In whatever form is appropriate, I’d like to know the facts before I make a decision.”

Republicans in the Senate control Trump’s fate. Should the House, led by Democrats, vote to impeach, the Senate would conduct a trial that would determine whether the president is removed from office.

House Republicans, meanwhile, are running interference for Trump, accusing the Democrats of launching another "witch hunt" that will cost them their majority in 2020.

The House Republicans most likely to give impeachment strong consideration are the approximately half-dozen centrists and possibly the larger group of nearly 20 who are retiring from Congress next year. More could follow.

“For the past three years, the socialist Democrats have been obsessed with impeaching the president and backfilling in the reason after the fact," said Minnesota Rep. Tom Emmer, chairman of the National Republican Congressional Committee, in a statement.