Democrats are laying out their case against President Trump for abuse of power as the Senate impeachment trial continues for a third day on Thursday.
Mr Trump should be removed from office for attempting to solicit election help from Ukraine, Democrats say.
The alleged abuse "threatens the entire system" of US democracy, argued Jerry Nadler, a senior Democrat.
Mr Trump is also charged with obstruction of Congress.
He denies both charges.
Mr Nadler and other Democratic members of the House of Representatives chosen to act as prosecutors in the impeachment trial have 24 hours over three days to make the case that the Senate should remove Mr Trump from office.
Oral arguments began on Wednesday, when the Democrat's lead manager Adam Schiff laid out an exhaustive case centred on Mr Trump's conduct toward Ukraine.
Mr Trump's defence team is expected to begin their rebuttal on Saturday.
The president was impeached by the Democrat-led House of Representatives, who approved the two charges in December.
A two-thirds majority vote in the Republican-controlled Senate is needed to remove him. Though none of the 53 Republican senators have said they will vote against Mr Trump, a handful are being carefully watched for signs of shift.
On his way in to the Senate on Thursday, Republican Mitt Romney that he would not answer questions about the ongoing proceedings.
"I'm really not going to comment on evidence or the process until the trial is completed," the Utah Senator said.
Mr Romney is one of three Republican senators who have indicated support for including witnesses and evidence in the trial - a point pushed by Democrats.
So far, Mr Schiff and the six other House impeachment managers have emphasised the allegation that Mr Trump withheld military aid and a White House meeting in order to pressure his Ukrainian counterpart to investigate former Vice-President Joe Biden - a domestic political rival.
"President Trump solicited foreign interference in our democratic elections, abusing the power of his office to seek help from abroad to improve his re-election prospects at home," Mr Schiff said. "If this conduct is not impeachable, then nothing is."
The Senate proceedings have proven contentious. After a heated exchange in the early hours of Wednesday morning, Chief Justice John Roberts, presiding over the Senate trial, scolded lawmakers on both sides.
"Those addressing the Senate should remember where they are," Mr Roberts said.
But in a show of rare bipartisan goodwill, Republican Senator Lindsey Graham - one of the president's fiercest defenders - was seen congratulating Mr Schiff on a job well done after his first day of arguments on Wednesday.
"Good job," Mr Graham was heard saying. "Very well spoken."