Democrats called US President Donald Trump a "cheat" who abused his power as they started laying out formal arguments to remove him from office on the second day of the impeachment trial.
The hours-long presentations by Democratic impeachment managers stretched into the evening on Wednesday, as an audience of 100 senators and Trump's defence team sat silently without access to mobile phones.
"President Trump solicited foreign interference in our democratic elections, abusing the power of his office to seek help abroad to improve his re-election prospects at home," said Adam Schiff, the lead impeachment manager.
The managers pointed to evidence gathered by House investigators that Trump and his surrogates leveraged a White House meeting and withheld nearly $US400 million ($A584m) in military aid to Ukraine, in a bid to pressure Kiev into announcing investigations into his political rival Joe Biden.
Schiff blasted Trump for attempting to "cheat" in the 2020 elections and then using the "unrivalled authority at his disposal as commander-in-chief to cover up his wrongdoing".
The managers have 24 hours over the span of three days to make their case against Trump, who is accused of abusing his presidential powers in pressuring Ukraine and then obstructing Congress.
Starting likely on Saturday, Trump's defence lawyers will be given an equal amount of time to present their case.
Trump called Schiff a "corrupt politician" on Wednesday, in comments made from a press conference at the World Economic Forum in Switzerland.
"They had no case. It's all a hoax. It's a con job," Trump said.
Trump is the third president in US history to be impeached and face the prospect of removal.
Two-thirds of the 100-member chamber are required to agree to remove a president from office. The Senate is controlled by Trump's Republican party, meaning the most likely outcome is an acquittal.
"The more we hear from Adam Schiff, the more the GOP is getting unified against this partisan charade!" Senator Rand Paul, a Republican from Kentucky, said on Twitter.
Trump has accused former vice president Biden of pursuing his son's interests by pressuring Ukraine to oust a prosecutor that had scrutinised a Ukrainian gas company. No evidence of wrongdoing by Biden has emerged.
Trump also asked Ukrainian President Volodymr Zelensky to investigate a debunked conspiracy theory that Ukraine meddled in the 2016 presidential elections.
Wednesday's arguments came after a session lasting more than 12 hours on Tuesday, which saw the Senate sparring over the conduct of the trial.
Republican leader Mitch McConnell was successful in rallying his majority against Democratic proposals to subpoena new witnesses and Trump administration documents.
Democrats decried the rules as an attempt to rush the trial, hide new evidence against Trump, and prevent witnesses from testifying.
On Wednesday, the impeachment managers played video clips of witness testimony and a clip of Trump publicly calling for Ukraine, along with China, to investigate Biden, who hopes to unseat Trump in the 2020 elections.
Schiff called the evidence an "overwhelming and damning picture" but added that there is also new evidence, and that unheard current and former top White House officials should testify to the Senate.
According to the trial rules, set by McConnell, witnesses will be considered by the Senate only after both sides make their opening arguments over the span of six days and the Senate is given 20 hours to ask questions and debate.
The impeachment trial of Trump was sparked by an anonymous whistleblower complaint, released in September, that detailed a backchannel led by Trump's personal lawyer Rudy Giuliani to pressure Ukraine into investigating the Bidens.
After investigating Trump's dealing with Ukraine, the Democrat-controlled House passed two articles of impeachment against the president in December, and sent the process over to the Senate for a trial.
Australian Associated Press