Republicans in Congress are predicting chagrin for the Democrats.
"I think they're going to be disappointed, just as they were when the Mueller report came out," said Republican Representative Tom Cole.
Mueller's report said the investigation found insufficient evidence to prove that Trump and his campaign engaged in a criminal conspiracy with Russia. The report also did not reach a conclusion on whether Trump committed the crime of obstruction of justice but pointedly did not exonerate him. Attorney General William Barr, a Trump appointee, subsequently cleared the president of obstruction of justice.
"It is so important for him to say that the investigation did not exonerate the president," said Representative Val Demings, another Democrat who sits on both panels.
Republicans are expected to raise questions with Mueller about the integrity of the origins of the Russia probe in the FBI during Democrat Barack Obama's administration and the political leanings of the special counsel's staff members. Trump has sought to portray the probe as an attempted coup.
The Justice Department on Monday sent a letter telling Mueller to limit his testimony to merely discussing the public report, a directive that the Judiciary Committee chairman rejected as out of line.
But even if Mueller's testimony proves disappointing enough to deflate impeachment chances, Democrats expect their continuing investigations to produce revelations that could damage Trump's re-election prospects.
The heart of the Democratic strategy is to focus Mueller's testimony on several examples of potential obstruction by Trump, including his repeated efforts to get former White House Counsel Don McGahn to remove Mueller and then deny that he had been directed to do so.
Lawmakers will also emphasize attempts by Trump to his fired former Attorney General Jeff Sessions to redirect the Russia probe away from his 2016 campaign, and possible witness tampering in public statements seen to discourage his former campaign chairman Paul Manafort and his former personal lawyer Michael Cohen from cooperating with federal investigations.
Democrats have said the report includes shocking evidence of misconduct that would result in criminal charges against any other American. The Justice Department has a longstanding policy against bringing criminal charges against a sitting president.
