(CNN) A growing number of GOP senators are now acknowledging that President Donald Trump may have leveraged US military aid to Ukraine in exchange for an announcement of investigations that could help him politically -- but they contend that even that conduct does not warrant removal from office or hearing from additional witnesses.

Republicans are arguing that the latest reports -- that former national security adviser John Bolton's book manuscript says that Trump told him in August that he was withholding $391 million in aid until Ukraine announced a probe into the Bidens -- are likely true but simply confirm what is already known.

And they are saying that new allegation, first revealed by The New York Times, is consistent with the details laid out by House Democratic managers in their case that Trump used official acts to urge a foreign power to undercut a leading political rival in the 2020 presidential campaign.

But they say that nothing in there is impeachable -- nor does it warrant the hearing from new witnesses since it confirms what is already known, they say. Yet it still remains to be seen if four Republicans break ranks to support witnesses, giving Democrats enough support that would dramatically change the course of Trump's trial.

"I don't think anything he says changes the facts," South Dakota Sen. John Thune, the majority whip, told CNN. "I think people kind of know what the fact pattern is. ... There's already that evidence on the record."

Sen. Kevin Cramer, a North Dakota Republican, added: "I think he sounds like a lot of the other witnesses, frankly. I don't know that he's got a lot new to add to it."

The comments are a departure from the arguments made by House Republicans, who disputed the central facts of the Democrats' case and said they failed to prove any link to Trump's official acts -- including withholding a White House meeting with the Ukrainian president along with the military aid the country sought to combat Russia.

On the Senate floor this week, the President's attorneys continued to argue that the Democrats lacked sufficient evidence to make their charges. But they said that even if they were true, the conduct is not impeachable, an argument many Senate Republicans are now echoing.

Republicans, in particular, are pointing to legal arguments by Trump attorney Alan Dershowitz, who said that Bolton's allegations -- if true -- are not impeachable.

"Nothing in the Bolton revelations -- even if true -- would rise to the level of an abuse of power or an impeachable offense," Dershowitz said.

"Even if the President, any president, were to demand a quid pro quo as a condition to sending aid to a foreign country, obviously a highly disputed matter in this case, that would not by itself constitute an abuse of power," he argued, adding, "Quid pro quo alone is not a basis for abuse of power, it's part of the way foreign policy has been operated by presidents since the beginning of time."

On Tuesday, Republicans said that argument was reason enough to quickly acquit the President -- and reject the calls for additional witnesses, since they said that Bolton in particular would add little to the record.

"I don't think the testimony of Ambassador Bolton would be helpful because I basically am in agreement with the very scholarly approach that Mr. Dershowitz took that there's no article there that's grounds for impeachment and removal," said Mississippi Sen. Roger Wicker, chairman of the Senate Commerce, Science and Transportation Committee.

Asked if anything in the Bolton revelations amounted to impeachable conduct, Sen. Roy Blunt of Missouri, a member of the GOP leadership, said: "I wouldn't think so -- for the reasons that were described yesterday on the floor."

"No," Republican Sen. Tim Scott of South Carolina said bluntly when asked if he thinks what Bolton is reportedly detailing amounts to impeachable conduct. "I don't think it would be."

Whether that strategy works remains to be seen. Just four Republican defectors are enough to prompt witness testimony in the trial, something that would throw the proceedings into uncertain and risky territory for the President. Fifty-one votes are needed to move ahead on witnesses, a vote expected on Friday.

"Mr. Bolton probably has some things that would be helpful for us and we'll figure out how we might be able to learn that," Sen. Lisa Murkowksi, an Alaska Republican and key swing vote, said Tuesday.

As they acknowledge that Trump may have done exactly what House Democrats are alleging, several Republicans are sidestepping questions about whether they can defend his handling of matters with Ukraine.

Asked if he had any concerns about Trump's conduct, Wicker said that's not the question before the Senate.

"Do I agree with everything every president has done? No. Does that include this President? Yes," Wicker said. "But the question is not whether, I think, a phone call was perfect, or whether something was advisable or not. The question is: Is this an impeachable offense?"

Texas Sen. John Cornyn added: "If you are going to start impeaching presidents for doing what's within their power but for which they have political benefit, than I think impeachment is going to become the new norm."

"The charges at the extreme do not rise to the level of impeachment," said Sen. Thom Tillis, a Republican from North Carolina who is up for reelection and vulnerable in 2020.

GOP leaders argue there's little reason to secure witness testimony since the outcome -- Trump's acquittal -- is inevitable.

Indeed, Thune suggested that hauling in Bolton would add little to what is already known, and that it wasn't worth the cost given that it could lead to a protracted fight for even more witnesses, like acting White House chief of staff Mick Mulvaney and Secretary of State Mike Pompeo -- and potentially end up in the courts.

"He could reinforce (the existing record) or put a different context on it," Thune said of Bolton. "But if we call him in then the Democrats will want to call other people in -- Mulvaney or Pompeo -- then our side would want to call more people in."

Thune added: "And I think that gets us into this endless cycle and this drags on for weeks and months in a middle of a presidential election where people are already voting."