But Mr. Trump and his Republican allies have sought to turn the tables by threatening to call Democrats, including the Bidens, to testify. That could allow Mr. Trump to amplify the same unfounded accusations against the Bidens that he was trying to get the Ukrainian president to agree to investigate in their July 25 phone call.
“We will have Schiff, the Bidens, Pelosi and many more testify, and will reveal, for the first time, how corrupt our system really is,” Mr. Trump tweeted this month before his impeachment, referring to Representative Adam B. Schiff of California, the chairman of the House Intelligence Committee.
The Senate, which is controlled by Republicans, could have difficulty calling Mr. Biden to testify, as some moderate party members might balk at the idea. The party holds 53 seats in the chamber and would need 51 votes to compel the former vice president to appear.
There is no evidence that the elder Mr. Biden, while serving as vice president, improperly intervened in Ukraine to benefit his son. But Hunter Biden was a board member of a Ukrainian energy company while his father worked on Ukraine policy under the Obama administration, a connection that some administration officials viewed as problematic.
On another front in the 2020 campaign on Friday, Senator Elizabeth Warren’s campaign disclosed in a fund-raising email that she had raised just over $17 million since Oct. 1, lagging her fund-raising pace from the third quarter of the year, when she raised $24.6 million.
“We’re only days away from the biggest fundraising deadline of the year, and we’re at risk of missing our $20 million goal,” her campaign website said.
Ms. Warren, of Massachusetts, is working to reinvigorate her campaign in Iowa after her national polling numbers plateaued in recent weeks. She is scheduled to hold several events in the state over the weekend.
Mayor Pete Buttigieg of South Bend, Ind., who is seen as a front-runner in Iowa, is also campaigning in the state over the next three days.