Donald Trump and White House lawyers pushed back on Wednesday against Democratic efforts to have John Bolton testify at the Senate impeachment trial of the president, arguing that the former national security adviser could reveal government secrets.

Senate Democratic Minority Leader Chuck Schumer said it may be an "uphill" fight to garner enough Republican support to compel the 71-year-old Bolton -- seen as a potentially damaging witness to the president -- to testify.

Former national security advisor John Bolton
Former national security advisor John Bolton (l) says President Donald Trump wanted to freeze military aid to Ukrain until Kiev opened an investigation into Joe Biden, the frontrunner for the Democratic presidential nomination. Photo: AFP / Brendan Smialowski

Democrats have been pressing for the Senate to subpoena Bolton, who was fired by Trump in September, after reports that his upcoming White House tell-all book corroborates charges that the president withheld military aid to Ukraine in a bid to seek political dirt on his election rival Joe Biden.

Republicans hold a 53 to 47 seat edge in the Senate and four Republicans would need to side with the Democrats for Bolton and others to be called as witnesses at Trump's trial for abuse of power and obstruction of Congress.

"We've always known it will be an uphill fight," Schumer said of getting Bolton to testify.

Senate Majority Leader Mitch McConnell
Counting votes: Senate Majority Leader Mitch McConnell (R) has told his party they might not be able to block calling witnesses in the impeachment trial of President Donald Trump. Photo: GETTY IMAGES NORTH AMERICA / MARK WILSON

"Is it more likely than not?" the Democratic senator from New York asked. "Probably, no. But is it a decent, good chance? Yes."

Trump called on Republicans to reject a push for witnesses when the issue comes up for a vote on Friday and blasted Bolton.

"Remember Republicans, the Democrats already had 17 witnesses, we were given NONE!" he wrote on Twitter. "Don't let the Dems play you!"

Senator Lisa Murkowski of Alaska
Senator Lisa Murkowski of Alaska is one of four Republicans that Democrats hope can be persuaded to vote for witnesses in the impeachment trial of President Donald Trump. Photo: AFP / Mandel NGAN

Senator John Barrasso of Wyoming came out firmly against calling witnesses, echoing the views of the vast majority of his Republican colleagues.

"America has heard enough," Barasso said. "A majority of Americans are saying this is a waste of time because they know the president is not going to be removed."

Chuck Schumer
Chuck Schumer, seen here arriving at the Capitol during the impeachment trial of President Donald Trump,is the top Democrat in the Senate. Photo: AFP / Mandel NGAN

Opinion polls on whether Trump should be removed are actually pretty evenly divided but a two-thirds majority -- 67 senators -- is needed to convict the president and the Republicans' 53-47 majority virtually assures Trump will be acquitted.

US Supreme Court Chief Justice John Roberts
US Supreme Court Chief Justice John Roberts reads a question from a senator at President Donald Trump's impeachment trial. Photo: US Senate TV / HO

The fight over Bolton intensified as the senators who hold the president's fate in their hands began directly questioning Democratic prosecutors and White House lawyers.

The 100 members of the Senate have 16 hours over the next two days to submit written questions, which are being read aloud by Supreme Court Chief Justice John Roberts, presiding over just the third impeachment trial of a president in US history.

Asked whether Bolton should appear, chief Democratic prosecutor Adam Schiff said his testimony was "plainly relevant" and essential to conducting a "fair trial."

Former national security advisor John Bolton
Profile of former US national security advisor John Bolton. Photo: AFP / Jonathan WALTER

"Don't wait for the book," the California lawmaker told the senators sitting as trial jurors. "This case is overwhelmingly clear without John Bolton but if you have any question about it you can erase all doubt."

White House deputy counsel Patrick Philbin countered that the Democratic-controlled House of Representatives, which impeached Trump on December 18, had conducted their inquiry in a "half-baked partisan fashion" and had not issued their own subpoena for Bolton's testimony.

Having Bolton testify would present "grave security issues," Philbin said.

"He has all of the nation's secrets from the time he was national security advisor," he said.

The White House would seek to prevent his testimony by citing executive privilege and the matter would end up in the courts. "This institution will be effectively paralyzed for months on end," he said.

Schiff pushed back against White House arguments that the Senate should let American voters decide Trump's fate in November's presidential election.

"If you say you can't hold a president accountable in an election year where they are trying to cheat in that election then you are giving them carte blanche," he said.

In his book, Bolton reportedly writes that the president told him in August that a freeze in military aid to Ukraine was directly linked to his demand that Kiev investigate Biden, the frontrunner for the Democratic presidential nomination.

Trump lashed out at Bolton on Twitter, saying he had shown poor judgment in a job he had begged for and that he hired the veteran diplomat and prominent "hawk" against the advice of others.

"Frankly, if I listened to him, we would be in World War Six by now," Trump said. "(He) goes out and IMMEDIATELY writes a nasty & untrue book. All Classified National Security. Who would do this?"

The National Security Council said meanwhile that a preliminary review of the manuscript of Bolton's book had found that it contained "significant amounts of classified information" and cannot be published in its current form.

"Some of this information is at the TOP SECRET level," the NSC said in a letter to Bolton's lawyer, adding that "the manuscript may not be published or otherwise disclosed without the deletion of this classified information."

Before the explosive revelations about what Bolton knew, the White House had assumed the trial could be wrapped up with no witnesses and an acquittal by this weekend, just in time for Trump's annual "State of the Union" address before Congress on Tuesday, February 4.

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