Democrats worked methodically at U.S. President Donald Trump's impeachment trial on Thursday to dismantle his long-standing allegation that Democratic presidential contender Joe Biden acted improperly toward Ukraine while vice president.

On the second day of their arguments for Trump's removal from office, Democratic House of Representatives members argued that Biden did nothing wrong and was only carrying out official U.S. policy when he pressured Ukraine to fire its top prosecutor, Victor Shokin, because of corruption concerns.

Trump and his allies maintain that Biden wanted Shokin out in order to head off an investigation into a natural gas company, Burisma, where his son Hunter served as a director. Democrats said no evidence supported that allegation.

In more than eight hours of argument, House managers spent Thursday focusing on the charge that Trump abused his office by pressing Ukraine to investigate the Bidens purely for political gain.

They contended that Trump pushed for the probe because he was worried about facing the former vice president in November's election. Biden is the front-runner for the Democratic presidential nomination.

"If we allow this gross abuse of power to continue, this president would have free rein – free rein – to abuse his control of U.S. foreign policy for personal interest, and so would any other future president," U.S. Representative Sylvia Garcia said on the Senate floor. "And then this president, and all presidents, become above the law."

Democrats contend senators should convict Trump on two charges brought by the Democratic-led House - abuse of power and obstruction of Congress.

But the Senate, which is controlled by Trump's fellow Republicans, remains unlikely to do so. A two-thirds majority is needed to remove him from office.

Even so, Democrat Adam Schiff, the lead House manager, ended the evening by commending the senators who had endured a long day of arguments.

"I just want to thank you keeping an open mind about all of the issues that we are presenting," Schiff said. "That's all that we can ask for."

The U.S. Constitution sets out the impeachment process for removing a president who commits "high crimes and misdemeanors." Trump's legal team has argued that the House charges were invalid because impeachable offenses must represent a specific violation of criminal law.

Trump condemned the proceedings as "unfair and corrupt" in a Twitter post on Thursday.

The charges against Trump arise from his request in a July 25 phone call last year that Ukraine investigate Biden on unsubstantiated corruption allegations and the president's actions to impede a House inquiry into the matter.

Trump also asked Ukrainian President Volodymyr Zelenskiy to investigate a discredited theory beneficial to Russia that Ukraine worked with Democrats to hurt Trump in the 2016 U.S. election. Trump temporarily withheld 391 million U.S. dollars in U.S. military aid to Ukraine, which Democrats say was leverage for his demands.