Updated June 29, 2016 - 6:09PM ET

Senate Majority Leader Mitch McConnell says he has reservations about the candidate at the top the 2016 Republican ticket: Donald Trump.

McConnell made headlines earlier this week for declining to say whether Trump is qualified for the presidency.

In an interview with Washington, D.C. bureau reporter Geoff Bennett, McConnell said Trump is not yet a credible presidential candidate.

Sen. Mitch McConnell: "Trump clearly needs to change, in my opinion, to win the general election. What I’ve said to him both publicly and privately: 'You’re a great entertainer. You turn on audiences. You’re good before a crowd. You have a lot of Twitter followers. That worked fine for you in the primaries. But now that you are in the general, people are looking for a level of seriousness that is typically conveyed by having a prepared text and Teleprompter and staying on message.' So my hope is that he is beginning to pivot and become what I would call a more serious and credible candidate for the highest office in the land."

Geoff Bennett: "At the moment, though, I hear you saying he does not meet that threshold of credibility?"

McConnell: "He’s getting closer. Getting closer."

During the interview, the Kentucky Republican complimented his former Senate colleague -- now Trump’s Democratic opponent -- Hillary Clinton.

Geoff Bennett: What was your working relationship like with Hillary Clinton when she was in the Senate?

Sen. Mitch McConnell: Fine. Yeah. She’s an intelligent and capable person, no question about it.

Bennett: If she happens to win in November and Republicans keep control of the Senate, do you think she’s someone you could work with?

McConnell: Well, I’m hoping that doesn’t happen. But we'll work with whoever's elected president.

That could include filling the vacancy on the Supreme Court. McConnell says he will not under any circumstances consider President Obama's nomination of Merrick Garland to replace the late conservative justice Antonin Scalia.

"I don't think the seat would have been filled with a vacancy created in a presidential year no matter who was in the White House," McConnell said. "And for those who argue that this is a disadvantage to the Supreme Court, I would remind them that only four cases this whole term since Scalia passed away were deadlocked. Only four."

In a Twitter post Wednesday afternoon, Trump disagreed with McConnell's characterization of Clinton as "capable."

McConnell spoke with Time Warner Cable News, in part, to promote his new memoir, "The Long Game."

The full interview is airing on Time Warner Cable News' Kentucky station, CN2.