Republican senator Lamar Alexander said he will oppose calling more witnesses, dashing Democrat Party efforts to hear more testimonies.
The announcement came shortly after Republican senator Susan Collins said she would vote to allow witnesses in the impeachment trial, building momentum for the Democrats’ effort.
It would take four Republican senators to break with the majority and join with Democrats to tip the outcome.
However Mr Alexander said in a statement there was “no need for more evidence”, giving the Trump team the likelihood of a senate vote in its direction.
A vote on the witness question, expected Friday, could lead to an abrupt end of the trial with Mr Trump’s expected acquittal.
Or it could bring days, if not weeks more argument as Democrats press to hear testimony from former national security adviser John Bolton and others.
Ms Collins said in a statement: “The most sensible way to proceed would be for the house managers and the president’s attorneys to attempt to agree on a limited and equal number of witnesses for each side.
“If they can’t agree, then the senate could choose the number of witnesses.”
But Mr Alexander said that “there is no need for more evidence to prove something that has already been proven and that does not meet the US Constitution’s high bar for an impeachable offence”.
Another Republican senator, Mitt Romney, has made clear he will vote for witnesses.
Mr Trump was impeached by the house last month on charges that he abused his power like no other president, jeopardising Ukraine and US-Ukraine relations.
Democrats say Mr Trump asked the vulnerable ally to investigate Joe Biden and debunked theories of 2016 election interference, temporarily halting American security aid to the country as it battled Russia at its border.
The second article of impeachment says Mr Trump then obstructed the house probe in a way that threatened the nation’s three-branch system of checks and balances.