The South Carolina senator has repeatedly argued against calling witnesses to speak on the charge of incitement to insurrection faced by the former president, who is accused of encouraging his supporters to attack the US Capitol on 6 January in violent scenes that left five dead.
Mr Graham, appearing on Fox News on Monday evening, warned Democrats against “opening that can of worms” of calling witnesses, arguing it would drag the trial out in a process that would be “bad” for America.
“To my Democrat colleagues, if you vote to call one witness, [when] none were called in the House, get ready for a long trial,” said the Republican senator.
“If you open that can of worms, we’ll want the FBI to come in and tell us about how people pre-planned this attack and what happened with the security footprint at the Capitol. You open up Pandora’s Box if you call one witness,” he said.
Mr Trump’s impeachment trial is scheduled to start the week of 8 February.
Mr Graham, one of the former president’s chief supporters among Republican leaders in the post-election period, last week said that the Democrats are turning the impeachment trial into a “circus” after the lawyer of the fur-donning Capitol rioter known as the “QAnon Shaman” said his client is willing to testify.
“The House impeached President Trump without a witness,” Mr Graham said in a tweet. “If we open the witness door in the Senate, there will be lots of witnesses requested on a variety of topics. And the trial will go for months, not days.”
As the former president await his second impeachment trial next week, Democrats have said they will at a minimum be using video footage from the riots, filmed by the rioters themselves.
Meanwhile, Mr Trump is still struggling to assemble a legal defence, with Wednesday being his deadline to respond to the article of impeachment. Hours before the deadline, Mr Trump announced a change in his team and hired two new lawyers after his five-person team abruptly resigned.