U.S. Republican Senator Mitt Romney said on Monday that it was "increasingly likely" at least four Republican senators would join Democrats in calling for John Bolton, the former White House national security adviser, to testify in the Senate impeachment trial of U.S. President Donald Trump.

The U.S. Senate is holding an impeachment trial on charges by the House of Representatives that President Donald Trump abused his powers and obstructed a congressional inquiry into his effort to pressure Ukraine to investigate Democratic presidential contender Joe Biden.

U.S. President Donald Trump's impeachment trial enters a pivotal week on Monday as his lawyers resume their defense following a fresh report that could intensify pressure on Senate Republicans to call former national security adviser John Bolton to testify.

Prominent attorney Alan Dershowitz and two former independent counsels, Ken Starr and Robert Ray, will be among those defending President Donald Trump when they make their case to the U.S. Senate on Monday.

Lawyers representing U.S. President Donald Trump in his Senate impeachment trial launch a second day of defense arguments on Monday as the proceedings on whether to remove him from office enter a critical week.

President Donald Trump's impeachment trial may not result in his removal from office, but it could help determine whether his Republicans retain control of the Senate in the November congressional elections.

The leading three Democratic U.S. senators running for president barnstormed across Iowa this weekend, seeking to maximize a frenzied 36 hours before returning to Washington to resume duties as jurors in President Donald Trump's impeachment trial.

Jerrold Nadler, one of the U.S. House lawmakers leading efforts to remove President Donald Trump from office said on Sunday he would miss some of the Senate impeachment trial after his wife was diagnosed with cancer.

U.S. President Donald Trump said on Sunday that the Democratic lawmaker leading the impeachment case against him, Representative Adam Schiff, has "not paid the price, yet" for his actions, a statement Schiff said he viewed as a threat.

Just before his lawyers stepped into the U.S. Senate to begin their argument that he should not be removed from office, President Donald Trump offered a preview of sorts.