Other Republicans tried to downplay Bolton's claims, saying they presented nothing new or weren't to be automatically trusted.
Asked if believes Bolton, Sen. John Kennedy of Louisiana said, "I wouldn't bet my house on it" and criticized the media frenzy surrounding the impeachment trial.
"I think everybody ought to pop a Zoloft, take their meds, and let's wait and finish up," Kennedy said.
Speaking to reporters before the trial resumed, Sen. Charles E. Schumer, the Senate minority Leader, called leaks about Bolton's book "stunning."
"It goes right to the heart of the charges against the president," said Schumer, D-N.Y. "Ambassador Bolton essentially confirms the president committed the offenses charged in the first article of impeachment.
"We're all staring a White House cover-up in the face," Schumer said, adding that if Senate Republicans decide against calling witnesses, "they're going to be part of the cover-up too."
Trump repeatedly denied the allegations, telling reporters at the White House that they were "false."
"I NEVER told John Bolton that the aid to Ukraine was tied to investigations into Democrats, including the Bidens," he tweeted shortly after midnight. "In fact, he never complained about this at the time of his very public termination. If John Bolton said this, it was only to sell a book."
The president's team has staked its defense in part on denying any link between the events at the heart of the impeachment case, which alleges that Trump abused his power by pressuring a foreign government to help his reelection campaign -- and then obstructed Congress by ordering aides not to testify or provide documents.
Trump's lawyers also have repeatedly declared that no witness had firsthand knowledge of a direct link between the White House order to withhold $391 million in congressionally authorized security aid and Trump's push for Ukraine to announce an investigation of Biden, who is running for the Democratic presidential nomination, and his son.