U.S. Senate Majority Leader Mitch McConnell (Front) heads to the Senate Chamber before the start of the Senate impeachment trial on Capitol Hill in Washington D.C., the United States, on Jan. 21, 2020. The impeachment trial against U.S. President Donald Trump kicked off Tuesday in the Senate as the chamber debated, and would later vote on, a resolution stipulating rules guiding the process. (Photo by Ting Shen/Xinhua)

The Republican-controlled US Senate voted early on Wednesday on party lines to approve the rules for President Donald Trump's impeachment trial, rejecting Democratic efforts to obtain evidence and ensure witnesses are heard.

As the third presidential impeachment trial in US history began in earnest, Trump's chief legal defender argued the Democratic case was a baseless effort to overturn the 2016 election but a top Democratic lawmaker said there was "overwhelming" evidence of wrongdoing.

Trump was impeached last month by the House of Representatives on charges of abuse of power and obstruction of Congress for pressuring Ukraine to investigate former Democratic Vice President Joe Biden, a political rival, and impeding the inquiry into the matter. The president denies any wrongdoing.

After US Chief Justice John Roberts convened the proceedings, the two sides began more than 12 hours of squabbling that lasted into Wednesday morning over Senate Majority Leader Mitch McConnell's proposed rules for the trial.

Senators voted along party lines, 53-47, to block four separate motions from Democratic leader Chuck Schumer to subpoena records and documents from the White House, the State Department, the Defense Department and the Office of Management and Budget related to Trump's dealings with Ukraine.

By the same tally, senators also rejected requests for subpoenas seeking the testimony of acting White House Chief of Staff Mick Mulvaney, former national security adviser John Bolton, White House aide Robert Blair and White House budget official Michael Duffey.

Under McConnell's hastily revised set of procedures for the trial, there will be 48 hours of opening arguments - 24 hours for each side - over six days, easing off an earlier plan to keep them to two days each. It also allows the House's record of the probe to be admitted as evidence.

The arguments began when the trial resumed at 1 pm (local time) on Wednesday.

Republican senators have not ruled out the possibility of further testimony and evidence at some point after opening arguments and 16 hours of senators' questions, but they held firm with Trump's lawyers to block Tuesday's Democratic requests for witnesses and evidence - a potentially good sign for the White House.

White House counsel Pat Cipollone, who is leading Trump's defense, attacked the foundation of the charges against the Republican president and said Democrats had not come close to meeting the US Constitution's standard for impeachment.

"The only conclusion will be that the president has done absolutely nothing wrong," Cipollone said, arguing in favor of McConnell's proposal to wait until later in the trial to decide whether to allow further witnesses or documents.