WASHINGTON — Ohio Republican Sen. Rob Portman said Sunday that, despite infighting in the GOP and dismal approval ratings for President Donald Trump, the party remains in "pretty good shape" and suggested that the president should stop being "defensive" about the investigation into Russian meddling in the 2016 election.

Noting Republican dominance in Congress and in statehouses around the country, the Ohio senator said during an interview on NBC's Meet the Press that divisions between the Republican grassroots and the establishment are nothing new.

But Portman also said Trump has been "too defensive" in his reaction to the Russia investigation while saying that lawmakers should keep pushing to uncover the facts of the case. "He won the election fair and square, he's duly elected and we ought to instead focus on the outrage that the Russians meddled in our election."

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"We need to get to the bottom of it and we need to go where the facts lead us," he added.

Portman, who announced before the 2016 election that he would vote for Mike Pence for president rather than for his party's nominee, added that Republicans should work together to accomplish policy goals like tax reform rather than focusing on "the tweets."

"I don't agree with every tweet," he said. "As you know, I've spoken up on occasion. Yet if you're focused on the tweets and not focused on actually accomplishing on what people are actually looking to have happen for them and their family, I think you are getting out of touch with the American people."

"I was not someone who at the end was able to vote for him, but when he was elected I said, 'I'm going to work with him," Portman said of the president.

Trump remains popular within his own party but registered the lowest job approval of his presidency in a new NBC News/Wall Street Journal poll released Sunday morning. Just 38 percent of Americans say they approve of the job the president is doing, although 81 percent of Republicans give him a thumbs up.

Related: Trump Approval Drops to Lowest Level Yet in NBC News/WSJ Poll

Portman also said Sunday that he's optimistic that the GOP tax plan will spur sufficient economic growth to offset its impact on the deficit.

"It's going to result in more investment coming here to this country, more economic activity," Portman said. "Everybody who looks at this tax reform proposal will be able to say, I think across the spectrum, this is going to change behavior, and it's going to change behavior in a way that encourages more job creation and economic growth."