WASHINGTON (Reuters) - The impeachment trial of U.S. President Donald Trump is nearing a pivotal vote on whether to hear from witnesses requested by Senate Democrats arguing for his removal from office.

U.S. President Donald Trump speaks at a campaign rally in Wildwood, New Jersey, U.S. January 28, 2020. REUTERS/Leah Millis

Whatever happens, Trump is unlikely to be removed because his Republican Party has a 53-47 majority in the Senate, and a two-thirds majority of the chamber is needed to remove a president. A decision to hear from witnesses would require only four Republicans to cross party lines, however.

WHEN IS THE VOTE ON WITNESSES?

It could happen as early as Friday. A Senate resolution laid out a two-step process for voting on whether to subpoena witnesses. First, there will be an up-or-down vote on whether to consider witnesses. If that passes there will be more votes on testimony from particular individuals.

WHY DOES IT MATTER IF WITNESSES TESTIFY?

A decision to hear from witnesses would drag out the trial and potentially undermine a key leg of Trump’s defense - that much of the Democrats’ case against him is based on hearsay.

One witness sought by Democrats, Trump’s former national security adviser, John Bolton, says in a new but as-yet unpublished book that Trump told him that a freeze on aide to Ukraine was linked to his push for the country to investigate former Vice President Joe Biden and his son, Hunter Biden, according to a New York Times report.

With an eye on Trump’s bid for a second term in the presidential election in November, many Republicans in the Senate have been pushing for a speedy trial without witness testimony.

Democrats argue that without witnesses there cannot be a proper trial. Republicans counter that Democrats should have called the witnesses during the impeachment inquiry in the House of Representatives. The White House had told officials not to cooperate in that inquiry.

If the Senate does not vote in favor of witnesses it is expected to quickly vote to end the trial and acquit Trump. Trump would then likely use his State of the Union address, scheduled for Feb. 4, to reset his political agenda.

There is one potential upside for Trump in a long trial with witnesses: Four Democratic senators vying for their party’s presidential nomination to face him in November — Bernie Sanders, Elizabeth Warren, Amy Klobuchar and Michael Bennet — would have to remain in Washington instead of campaigning across the country.

WHO ARE THE POTENTIAL WITNESSES?

Senate Democratic Leader Chuck Schumer wants to hear from four current and former Trump administration officials that Democrats say have first-hand knowledge of key decisions in the effort to pressure Ukraine to investigate Biden.

The most prominent of the four are Bolton and Mick Mulvaney, Trump’s acting chief of staff. They also want to hear from Robert Blair, a Mulvaney deputy, and Michael Duffey, a White House budget official who ordered the Pentagon to freeze $391 million in security assistance for Ukraine.

Republicans have said that if Bolton testifies that would open the way for them to call witnesses, such as Hunter Biden. Trump has alleged that when Biden was vice president he tried to have Ukraine’s then-chief prosecutor fired to stop him investigating Burisma, an energy company on whose board Hunter had a seat.

While this claim has been widely debunked and the Bidens have denied any wrongdoing, Democrats are worried that having Hunter testify could still hurt his father, who is a leading contender for the Democratic presidential nomination.

COULD TRUMP BLOCK BOLTON FROM TESTIFYING?

Trump has said he would use a legal doctrine called executive privilege to block or limit Bolton’s testimony.

Executive privilege, in certain situations, allows a president to keep private the nature of conversations with aides. The doctrine is rooted in the idea that a president governs more effectively if aides can speak candidly.

Legal experts say that Chief Justice John Roberts, who is presiding over the trial, or the senators themselves, would rule on any claim by Trump that his conversations with Bolton are protected by executive privilege.

Trump could also file a lawsuit seeking a court order that would limit Bolton’s testimony.

Some legal experts argue that Trump’s executive privilege claim would be weak, saying Congress’ interest in hearing from Bolton would outweigh Trump’s desire for secrecy.