Senate Majority Leader Mitch McConnell (R-Ky.) reportedly told his caucus that he did not have the votes to prevent the Democrats' demands for new impeachment trial witnesses.
The possible inclusion of new witnesses has been a contentious debate between Democratic critics of President Donald Trump and his Republican allies and supporters.
On Tuesday, the Wall Street Journal reported that McConnell admitted that enough Republicans had crossed over to support the demands from Democrats. However, many Republicans remained uncommitted on the issue.
Fox News confirmed the report but added that it was unclear just what the claim meant, since some Republicans are willing to make concessions in order to call Hunter Biden to testify.
Sen. Mitt Romney (R-Utah) said that he was among those Republicans who wanted to hear from new witnesses, including John Bolton, Trump's former national security adviser.
Bolton reportedly claimed that Trump was withholding military aid from Ukraine, a central accusation of the impeachment articles, in an upcoming book.
Rep. Mark Meadows (R-N.C.) warned Republicans publicly on Monday that if they voted with Democrats for new witnesses, they could face political repercussions.