Trump pressures GOP senators on witnesses after McConnell warns he needs votes

President Donald Trump, in a series of tweets Wednesday, pressured Republican senators to vote against calling new witnesses in his impeachment trial after Senate Majority Leader Mitch McConnell acknowledged that he didn't yet have enough votes to block them.

“No matter how many witnesses you give the Democrats, no matter how much information is given, like the quickly produced Transcripts, it will NEVER be enough for them,” Trump said. “They will always scream UNFAIR. The Impeachment Hoax is just another political CON JOB!”

Trump also resumed attacks on John Bolton, his former national security adviser, calling Bolton’s account of his time at the White House “nasty” and “untrue.”

The New York Times reported that in a new book, Bolton claims Trump told him he would keep withholding U.S. military aid to Ukraine until Ukraine agreed to help investigate former Vice President Joe Biden and his son Hunter.

McConnell and the GOP senators met behind closed doors Tuesday evening shortly after Trump's legal team ended their opening arguments, in which they tried to discredit Bolton's reported allegations.

GOP Sen. Susan Collins of Maine claimed credit Wednesday morning for shaping the initial impeachment procedures resolution to require a vote adding witnesses and additional evidence. A Senate vote on whether to consider calling witnesses and allowing other new evidence is expected as early as Friday.

“I am pleased that I along with Mitt Romney, Lisa Murkowski and others worked very hard to get into the resolution a guaranteed vote on whether or not to call witnesses at this point in the trial,” Collins said Wednesday.

Trump's tweets come after one of his lead lawyers, Jay Sekulow, also tried to dismiss and discredit the Bolton allegations, arguing Tuesday, "You cannot impeach a president based on an unsourced allegation."

He called the Bolton book manuscript "inadmissible."

The development landed like a bombshell amid Trump's trial, with Democrats insisting that Bolton now must be called as a witness and even some key moderate Republicans, including Sens. Mitt Romney of Utah and Collins, expressed support for the idea, but have not yet sided firmly with the Democrats.

Even before the trial began, Democrats have been targeting four Republican senators -- Romney, Collins and Sens. Lisa Murkowski of Alaska and Lamar Alexander of Tennessee -- to get to the 51 votes Democrats need to force the Senate to call witnesses, including Bolton.

A senior White House official told ABC News that the president's defense team still believes they will be able to defeat the measure to call witnesses.

"We are exactly where we were going into the weekend," the official said. "There are four senators in play, two of have spoken publicly about where they stand (Romney, Collins) and two who have not (Murkowski, Alexander)."

"It's still a hard vote, but we are working hard. It's a long time until Friday," the official added.

Still, the Senate's number two Republican, John Thune, who is responsible for whipping the vote on witnesses, acknowledged Tuesday to ABC News that there is genuine fear that the trial could turn into a chaotic mess.

"Nobody wants a wide-open, sort-of free-for-all where this thing gets bogged down for weeks on end," Thune said.

He said he thought the GOP conference was unified behind a plan that would see more witnesses called than the Trump team would want, in exchange for a witness like Bolton.

But Thune said it was proving difficult to figure out how to manage what could become an unwieldy process.

"My assumption is that the president's counsel is going to have a fairly long list that they'll want to call, if the Dems get to have the witnesses they want to call. So, I just think it's fraught with a lot of peril and could be a long, drawn-out process," said Thune.

ABC News Chief White House Correspondent Jonathan Karl contributed to this report.