The Kentucky Republican told The Associated Press in an interview Wednesday that that it would be "very, very difficult to unseat the president in the primary."
Republicans, such as retiring Sen. Jeff Flake of Arizona, are considering challenging Trump in 2020.
McConnell said that "would be a waste of time frankly. But the people who would be likely to challenge the president would not be coming to me for advice."
McConnell also said it's not his job to police Trump's Twitter habits or his divisive rhetoric at campaign rallies and elsewhere.
He cited the Trump administration's "extraordinary" success with the GOP-controlled Congress — including a growing economy, two new Supreme Court justices and congressional action to cut regulations and taxes.
McConnell said he "generally" declines reporters' requests for him to critique "on a daily basis" the president's broadsides of political opponents and the FBI, along with Trump's most incendiary comments about women, immigrants and other groups of Americans.
McConnell also warned Democrats to think carefully about "presidential harassment" if they win the majority of the House.
He said Democratic threats to investigate Trump's finances and businesses would "help the president get re-elected" in 2020.
He recalled how Republicans were enthusiastic about going after President Bill Clinton in the 1990s. The GOP-led House impeached Clinton but the Senate acquitted him. The public was not as supportive as Republicans had hoped.
McConnell says 'It worked exactly the opposite. The public got mad at us and felt sympathy for President Clinton."
McConnell added that "this business of presidential harassment may or may not quite be the winner they think it is."
