Mitch McConnell says he lacks the votes necessary to block attempts to introduce new witnesses in Donald Trump's impeachment trial, according to reports.

Republicans hold a 53-seat majority in the Senate and can't afford more than three votes to peel away from the Senate Majority Leader's attempts to bring the trial to a close.

The president's legal counsel delivered its closing arguments on Tuesday.

Mr McConnell's closed-door announcement arrives after former national security chief John Bolton, in a leaked manuscript of a forthcoming book, confirmed that the president froze military aid to Ukraine in an attempt to pressure the country's president to investigate Joe Biden's son, at the core of the abuse of power charge against Mr Trump.

Following his departure from the administration, Mr Bolton also said he would be willing to testify.

All the president's lawyers: The team fighting Trump's impeachment Show all 6 1 /6 All the president's lawyers: The team fighting Trump's impeachment All the president's lawyers: The team fighting Trump's impeachment Alan Dershowitz Dershowitz is a controversial American lawyer best known for the high-profile clients he has successfully defended. Those clients have included OJ Simpson, Jeffrey Epstein and Harvey Weinstein. One longtime Harvard Law associated told the New Yorker Dershowitz "revels in taking positions that ultimately are not just controversial but pretty close to indefensible." Getty All the president's lawyers: The team fighting Trump's impeachment Ken Starr Starr became a household name in the 1990s as the independent counsel who led the investigation that led to Bill Clinton's impeachment. That investigation began as a look into a real estate scandal known as Whitewater, and eventually led to impeachment after Mr Clinton lied under oath about having an affair with White House intern Monica Lewinsky. AP All the president's lawyers: The team fighting Trump's impeachment Jay Sekulow Sekulow is the president's longtime personal attorney, and, now, personal lawyer in the White House. He has been accused by former Rudy Giuliani associate Lev Parnas of being "in the loop" during the Ukraine scandal. Getty All the president's lawyers: The team fighting Trump's impeachment Pam Bondi Bondi is the former attorney general in Florida, and a longtime backer of the president's. She made a name for herself in Florida for taking hyper partisan stances on issues, and her penchant for publicity. She is likely to be a prominent public-facing figure during the trial. AFP/Getty All the president's lawyers: The team fighting Trump's impeachment Pat Cipollone Cipollone is the White House counsel, and leading the president's defence team. Getty All the president's lawyers: The team fighting Trump's impeachment Rudy Giuliani While not officially named as one of the president's impeachment lawyers, it is hard to ignore Giuliani's outsized role in this process. The former mayor of New York has been making headlines for months as he defends his client, and for his apparent role in the effort to compel Ukraine to launch the investigation into Joe Biden. We'll see how he figures in the actual trial, which he has said he would like to be a part of. Reuters

Several Republican senators have indicated that they would be interested in his testimony, following the publication of the manuscript. Senators who are undecided about whether to call witnesses will likely face intense scrutiny over the next several days.

The White House blocked members of the administration from testifying during the House investigation.

In defence against a charge of obstruction, the president's legal team has argued over the last several days that Mr Trump exerted his executive privilege by denying requested documents, and Mr Trump also has expressed national security concerns over high-ranking officials like Mr Bolton appearing as witnesses.