AMNA NAWAZ: Well, as Lisa and Laura reported, the stakes are high, not just for President Biden, but for Republicans, too, in this pivotal election year.

When John Thune of South Dakota first won his Senate seat 20 years ago, he made history by defeating a party's Senate leader for the first time in over 50 years.

Today, he is a candidate to replace Mitch McConnell as the Senate's top Republican.

I spoke with Senator Thune moments ago.

Senator Thune, welcome back to the "NewsHour."

How are you?

SEN. JOHN THUNE (R-SD): Thanks, Amna.

Good.

Nice to be with you.

AMNA NAWAZ: Thank you so much for joining us.

It's fair to say now your party essentially has a presumptive nominee in former President Trump after Super Tuesday.

You have endorsed him, as have many others in leadership.

I should point out you were among those criticizing him after his second impeachment, when you said what he did -- quote -- "to undermine faith in our election system and disrupt the peaceful transfer of power is inexcusable."

So why support him today if what he did then was inexcusable?

SEN. JOHN THUNE: Well, look, I said what I said then and I'm not going to relitigate it, but I think what we have in front of us now is a very clear choice.

And the voters, Republican voters around the country, have made clear that they want President -- former President Trump to be the Republican nominee.

And I have said I would support the Republican nominee, and I think now we have a clear choice.

It's either going to be Trump or Biden.

And my view is, we need to put an end to the Biden-Schumer agenda, which consists of higher prices, broken borders, less energy independence, and a weakened America on the global stage.

Those are things that a lot of us believe that we can change, if we can unite behind our nominee and get a majority in the United States Senate.

And those are the types of things we're going to be working on because we think the stakes are really high for the American people.

AMNA NAWAZ: It seems there are a number of voters who still have questions, though.

The primaries revealed specifically among Republican primary voters in Iowa, New Hampshire, and South Carolina anywhere between 61 and 76 percent of Nikki Haley supporters said that they'd be so dissatisfied with Mr. Trump that they would not vote for him in November.

So how does he win them over?

SEN. JOHN THUNE: I think it's really important for the former president, now that the -- a lot of those primaries are out of the way -- there are still some coming, but he clearly is winning very decisively.

He's got to reach out and build a coalition of support that is broad enough, wide enough, strong enough, and deep enough to win a national election.

He's got tremendous support, as you know, loyalty among base voters, but general elections are decided in the middle of the electorate.

And so I think it's really important that you reach out to Nikki Haley supporters, to suburban voters, independent voters, people who a lot of times are -- can swing back and forth in elections.

He's going to have to have them in order to win the election in November.

And a lot of us believe that he's got to perform that way at the top of the ballot, and it will help our downballot races in the Senate and the House.

And we're trying to get a majority of the United States Senate.

AMNA NAWAZ: I want to ask you about the race to replace Senator Mitch McConnell as leader when he steps down later this year.

You have thrown your hat in the ring.

On the issue of Ukraine aid specifically, do you think, as leader, you would be able to convince House Republicans to back aid to Ukraine in a way that Mitch McConnell has not yet so far, especially if Mr. Trump continues to oppose it?

SEN. JOHN THUNE: I think that the challenge, of course, right now, with the situation deteriorating there, is to try and feel a sense of urgency about this.

I'm hoping the House can execute on getting something passed, ideally the Senate bill, but if not something that they could send to the Senate that we could then act on.

I'm hoping that there is enough of a consensus and a majority of Republicans in the House who believe that we need to defend America's interests, we need to stand with our allies, that this does represent a national security threat for our country.

America cannot retreat from the world stage.

American leadership is desperately needed now more than I think any time in recent history, and we need to make sure that Ukraine has the weaponry and the resources that it needs to defeat the Russians, because, if we're not sending them American weapons, and they succeed in Ukraine, and they go into a -- roll into a NATO country, then we're going to be sending American sons and daughters.

And I would much rather send weaponry, ammunition, that sort of thing, and let the Ukrainian people, who have been really good about carrying the fight to the Russians, win that battle.

And I hope that we can get the political support to make that happen.

AMNA NAWAZ: Senator, as you know, abortion access remains a top issue for millions of voters.

We do expect President Biden to speak to that tonight.

And I guess, when you ask Democrats, what is their stance on abortion, they say protect a woman's right to choose.

What would you say is the Republican stance on abortion access?

SEN. JOHN THUNE: Well, I think that the Republican position is to let the people decide.

And what the court said is, this is not going to be decided by people in Black robes in Washington, D.C.

This needs to go back to the states and their voters.

And I think that's exactly what's happening.

States around the country are making decisions, putting policies in place that reflect the will and the sentiment of the voters in those individual states.

AMNA NAWAZ: On a related issue, we saw the Alabama ruling on IVF show just how far and wide-reaching the implications of the overturning of Roe v. Wade can go.

Your Republican colleague blocked a bill that would actually protect IVF access nationwide.

So I just wanted to ask on your stance.

Do you support IVF treatment?

SEN. JOHN THUNE: Absolutely.

IVF has been an amazing, I guess you would say, solution for a lot of couples and families that haven't been able to have kids.

And I think it's been made clear.

Of course, the Alabama court took a position, but the Alabama legislature has since ruled or proposed legislation on that, which the governor has since signed in support of IVF.

And I don't think that you will find very many people in this country who don't realize what an effective method it is of enabling people who otherwise couldn't be able to have a family.

So we're pro-family, we're pro-life and IVF represents that.

AMNA NAWAZ: I need to ask you more broadly about this moment in time where we are now eight months before the election.

You have two elderly, unpopular candidates with very different visions for America.

One, however, does face 91 criminal charges.

He tried to stop the peaceful transfer of power and already lost to Mr. Biden in 2020.

So, in an election that hinges on a few states and very narrow margins, are you worried that voters concerned about all of those things will either stay home or vote for a third-party candidate?

SEN. JOHN THUNE: I think, in the end -- and I know there's always a lot of conversations about third-party candidates.

And, clearly, in some cases, if people don't like the options, they could stay home.

But I do really think that you will see a big turnout this year because I think people see it a very clear choice.

And you pointed out these are contrasting visions and these are very different views about how to lead this country and the direction we ought to head in.

I'm one who believes that we need to pivot, we need to go in a different direction, and that we need new leadership both in the White House and in Congress, or at least in the Senate.

And so my view is that, as people think about this election, yes, I want them to vote.

I want the electorate to be energized and engaged.

And I ultimately believe they will be, because I think, as I said before, the stakes are really high.

And I think people understand what's at risk here if we don't get it right.

And I'm hopeful, again, that we will see people turn out in big numbers and that they will vote for Republicans for Senate and hopefully President Trump for the White House.

AMNA NAWAZ: That is Senate Republican Whip John Thune of South Dakota joining us tonight.

Senator, thank you.

Always great to see you.

SEN. JOHN THUNE: Thanks, Amna.

Nice to see you.

AMNA NAWAZ: And we will have more live coverage of the State of the Union online and later tonight beginning at 9:00 p.m. Eastern right here on PBS.