Senate Majority Leader Mitch McConnell blocked then-President Barack Obama's attempts late in his second term to place Merrick Garland, a federal judge, on the high court after the passing of Justice Antonin Scalia. But the GOP president signalled he and the Kentucky Republican would move swiftly to put a third conservative justice on the court if a seat opens, even as polls show Mr Trump trailing former Vice President Joe Biden.
"Absolutely, I'd do it. Sure," Mr Trump told radio host Hugh Hewitt on Tuesday. "Sure."
"I would move quickly. Why not?" he added.
Mr Trump, who views politics as a never-ending fight, described his plans as just because, in his view, the opposition would do the same thing if they controlled the White House and Senate.
"I mean, they would. The Democrats would, if they were in this position," he said. "But you know, I'll be interested."
The president has a point: Mr Obama tried to do just that, but failed to break through Mr McConnell's blockade.
Mr Trump has said recently that he is revising his list of conservative Supreme Court candidates, signalling he plans to release it before the 3 November election.
A Supreme Court vacancy before then has become even more charged because Democrats could take back the Senate.
Liberal Justice Ruth Bader Ginsburg is 87, and is a four-time cancer survivor. She announced earlier this year she is battling the disease again, this time on her liver.