Democrats, according to Schumer, are expected to force votes on the four witnesses, as well as three sets of documents on Tuesday — when the Senate takes up and debates the trial rules resolution.

Republicans have the 51 votes to pass their own rules resolution over the objections of Democrats. But the resolution is amendable, allowing Democrats to force votes to work in their requests.

Democrats believe a new Government Accountability Office (GAO) report , as well as a steady stream of news reports are adding pressure to Republicans to support calling witnesses and Ukraine-related documents.

The GAO issued a report on Thursday accusing the White House budget office of breaking the law by withholding military aid to Ukraine — the very issue at the heart of the Democrats' impeachment effort.