A fourth expert called by Republicans said there was not enough evidence to back up the impeachment allegations, suggesting Democrats should wait for judges to rule whether Bolton and other key officials like Chief of Staff Mick Mulvaney and Secretary of State Mike Pompeo need to appear.

Trump is alleged to have abused the power of his office by putting personal political gain over national security interests, engaging in bribery by withholding $400 million in military aid Congress had approved for Ukraine, and then obstructing Congress by stonewalling the investigation.

It’s still unclear how many articles of impeachment will be passed and whether they will only cover the Ukraine scandal.

One key question is whether Democrats will include evidence of Trump’s obstruction of justice unveiled by special counsel Robert Mueller.

That would broaden the case against Trump by portraying him as a lawless leader from virtually the day he took office. But some moderate Democrats are wary of including evidence from the Russia probe because they did not launch impeachment proceedings over those revelations when they took place.

Democrats also want to send a message to the nation that they are anxious to return to bread-and-butter issues as the 2020 vote looms, in which the president and all 435 House members will be up for election.