A senior Republican senator said momentum was building on Thursday for a quick end to President Donald Trump's impeachment trial despite a push by Democrats to call witnesses such as former national security adviser John Bolton.

White House: Bolton's book can't be out for national security reasons

John Barrasso, the Senate's third-ranking Republican, told reporters during the second day of questioning by U.S. senators that Republicans were likely to beat back the Democratic effort for witnesses and wrap up the trial on Friday.

"We still feel very positive about it," Barrasso said. "The momentum is on the side of having the final vote and final judgment tomorrow."

Trump's likely acquittal by the Republican-controlled Senate would leave him in office and allow him to claim vindication just as the Democratic Party holds its first nominating contest for the November 3 election in Iowa on Monday. Trump will hold a rally in the state on Thursday night.

Democrats accuse the Republican president of abusing his power by using congressionally approved military aid as leverage to get a foreign power to smear former Vice President Joe Biden, a leading contender for the Democratic nomination.

The Democratic prosecutors argue witnesses are essential to shed more light on Trump's attempt to persuade Ukraine President Volodmyr Zelenskiy to investigate Biden and his son Hunter, who sat on the board of Ukrainian energy company Burisma.

The Democratic-led House of Representatives approved in December the two articles of impeachment, including obstruction of Congress. Trump has denied wrongdoing and denounces the impeachment process as a sham.

(With input from Reuters)