Democrats on Wednesday accused Donald Trump at his historic Senate impeachment trial of seeking to cheat to ensure re-election in November, and called for "courage" by the president's fellow Republicans while considering the case against him.

Adam Schiff, head of the House of Representatives' prosecution team, took to the Senate floor to deliver hours of methodical arguments to a hushed chamber hearing only the third-ever impeachment trial of a US president.

The Democratic lawmaker described how Trump solicited foreign interference in domestic elections, "abusing the powers of his office to seek help from abroad to improve his re-election prospects at home."

"And when he was caught, he used the powers of that office to obstruct the investigation into his own misconduct," said Schiff, who headed the probe that led to Trump's December 18 impeachment by the Democratic-controlled House.

Trump stands accused of withholding military aid from Ukraine to pressure his Ukrainian counterpart to announce an investigation into Democrat and potential election rival Joe Biden.

"President Trump withheld hundreds of millions of dollars in military aid to a strategic partner at war with Russia to secure foreign help with his re-election," Schiff said at the nationally televised proceedings. "If this conduct is not impeachable, then nothing is."

Interspersing his remarks with video of House inquiry testimony, and clips of Trump himself, Schiff appealed to the Senate's 100 members to put aside partisanship in deciding Trump's fate.

He encouraged them to use an open mind when evaluating the testimony of people like ambassadors Gordon Sondland and Marie Yovanovitch, and former National Security Council official Fiona Hill.

"They risked everything, their careers. And yes I know what you're asked to decide may risk yours too," Schiff told the senators.

"But if they could show the courage, so can we," he said in concluding a nearly-nine-hour session.

Republicans, who hold a 53-47 edge, have shown little inclination, however, to break ranks with athe president.

Sixty-seven senators, a two-thirds majority, are needed to remove Trump from office. Republicans defeated repeated efforts by Democrats to introduce White House witnesses and documents at the start of the trial. Trump called the proceedings a "hoax" and said he expected the Senate to clear him "fairly quickly."