Republican Sen. Rob Portman, who did not vote for Donald Trump in 2016, told CNBC on Wednesday that he backs the president's reelection in 2020 because of the success of the White House's economic agenda.

"I'm going to vote for him, and I'm helping him, and I'm endorsing him. I cannot believe the change we have seen in terms of our economic growth, in terms of restoring our military strength. He's done a lot of great things," Portman said on "Squawk Box."

"You look at the policy front, I think the president is in pretty good shape to be reelected. And I'm very pleased obviously with a lot we've done in terms of the tax reform and not just the tax cuts but reforming our tax code," added the Ohio Republican, who first endorsed Trump's reelection bid late last month.

In 2016, Portman withdrew his support for Trump about a month before the election after the then-GOP presidential nominee was heard on a leaked 2005 "Access Hollywood" tape making lewd remarks about women.

At the time, Portman released a statement criticizing Trump for "offensive" comments. The senator said then he would not vote for then-Democratic nominee Hillary Clinton but instead would vote for Mike Pence for president. Pence, a former Indiana governor and ex-U.S. congressman, of course, was Trump's running mate who became vice president.

Wednesday on CNBC, Portman said that he wanted to look forward when asked about his past discomfort with Trump.

"I did endorse him [in 2016] and after the tape came out, you know, I said I was going to vote for Mike Pence instead," he said. "The point is going forward we got to figure out how to pull together as a country and solve some of these big problems."

Portman sees reducing prescription drug costs as a bipartisan issue.

"If you're [House Speaker] Nancy Pelosi and looking at the House majority and you want to get reelected what can you turn to as a real accomplishment? One would be prescription drugs; same as the Republicans over here," he said. "The president has said he wants to sign something."

Democrats control the House and Republicans hold the majority in the Senate. In addition to the presidential election, the balance of power on Capitol Hill is also up for possible change in the November voting.

Portman also views infrastructure as something both parties can get behind.

"There's a bipartisan interest in fixing our crumbling roads and bridges," he said. "The question is how do we pay for it. We're working on it."

Other areas of bipartisan interest, according to Portman, are how to tackle the nation's opioid crisis and how to train and retrain workers so they can get back into the labor force.