The House voted December 18 in favor of impeachment articles which charged President Trump with abuse of power and obstruction of Congress; but Pelosi held back on sending them to the Senate, in hopes of forcing Republicans to call witnesses who refused to testify before House impeachment investigators.
GOP leaders in the Senate refused to agree to anything along those lines, as Republicans said Pelosi had been forced to back down.
“She's caving and sending her Articles of Impeachment to the Senate without getting a single one of her absurd demands because her own party got fed up with her games,” said House Minority Whip Steve Scalise (R-LA).
As for what witnesses will be called, Senate Majority Leader Mitch McConnell has repeatedly noted that the witness list will be determined by a majority of Senators - 51 votes - who could decide to call anyone desired by either party.
Democrats - and some Republicans - have already indicated that they want to take up former Trump National Security Adviser John Bolton's offer to testify, as a small group of GOP Senators could hold the keys to who exactly comes before the Senate, or gives testimony in a deposition.
Those Senators include Susan Collins of Maine, Mitt Romney of Utah, Cory Gardner of Colorado, and Lisa Murkowski of Alaska.
News reports on Friday indicated that Collins is already working to broker some sort of witness compromise.
McConnell has indicated the Senate will first approve a resolution to set out rules to govern the first phase of an impeachment trial, which would include opening arguments, and written questions from Senators.
Here is a link to the similar resolution approved by Senators in 1999 for the start of the Clinton impeachment trial.