Impeachment manager Rep. Adam Schiff argues in favour of amendment during the impeachment trial in the Senate. AP

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.

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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.

Presiding officer Supreme Court Chief Justice John Roberts calls for a vote during the impeachment trial. AP

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.

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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.

Senate Majority Leader Mitch McConnell gestures as he walks back to the Senate chamber in Washington. AP

The arguments will begin when the trial resumes at 1 p.m. (1800 GMT) 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.

'No answers'

Democratic Representative Adam Schiff, who helped spearhead the House impeachment inquiry, said the president had committed "constitutional misconduct justifying impeachment."

While the evidence against Trump was "already overwhelming," further witness testimony was necessary to show the full scope of the misconduct by the president and those around him, he said.

White House adviser Pam Bondi argues against an amendment in the Senate at the US Capitol in Washington. AP

Republican senators have not ruled out the possibility of further testimony and evidence at some point after opening arguments and senators' questions.

But Democrats said they forced repeated votes on evidence and witnesses, which extended late into the night, to get Republicans on the record immediately.

"This may be our only chance, tonight, to make this a fair trial. And it's just increasingly clear that the White House has no answers for why these documents and these witnesses shouldn't be produced," US Senator Chris Murphy of Connecticut told reporters.

Under McConnell's plan, lawyers for Trump could move early in the proceedings to ask senators to dismiss all charges, according to a senior Republican leadership aide, a motion that would likely fall short of the support needed to succeed.

Even if such a motion fails, Trump is almost certain to be acquitted by the 100-member chamber, where a two-thirds majority is needed to remove him from office.

But the impact of the trial on his re-election bid in November is far from clear.