GWEN IFILL: And now to politics.

Four states are holding contests today, both parties in Mississippi and Michigan, plus Republican races in Idaho and Hawaii.

On the campaign trail, Republican John Kasich was in Michigan, drumming up support for today's primaries, while others, like Ted Cruz, turned their attention to states voting next week.

GOV. JOHN KASICH (R-OH), Republican Presidential Candidate: You know the whole country is watching Michigan now. The whole country is.

GOV. JOHN KASICH: And what they're wondering, what they're wondering, is a guy who has labored in obscurity rising in this state? And can a positive message of hope, of opportunity, of confidence, of innovation, of youthfulness, can somebody with that message finally overcome the bad mood that we seem to have been in?

SEN. TED CRUZ (R-TX), Republican Presidential Candidate: To all of you who are voting today in states across the country, who will be voting in a week, to all of the men and women here in North Carolina who may be thinking of supporting other candidates, I ask you, come join us. We welcome you with open arms. If you don't want to see Donald Trump as the nominee, and if you don't want to see Hillary Clinton as the president, which is the inevitable result of Donald Trump being the nominee, come join us. Let us stand together.

GWEN IFILL: The top prize for both parties tonight, Michigan. For some GOP candidates there, the fight for first is almost as meaningful as the fight for second place.

And for more on that, we're joined by Kathy Gray, political reporter for The Detroit Free Press.

Kathy, thank you for joining us.

In a nutshell, why is Michigan so important to these candidates?

KATHY GRAY, Detroit Free Press: Well, it propels them into the next states, Ohio, Florida, the big states on October (sic) 15.

And all of them are hoping that they get some big numbers here so they have some momentum going into those states.

GWEN IFILL: We have seen the Trump steamroller going through a lot of the early primary states. How is it going? How is it rolling through Michigan?

KATHY GRAY: Well, he has been here twice. He had big, big rallies in Michigan, and all the polls show him in the lead.

But some of the polls in more recent days have had the race tightening up a little bit. John Kasich seems to be rising a little bit. Ted Cruz is in second place. Marco Rubio seems to be kind of dropping off the — dropping off the slate.

GWEN IFILL: Some of the early exit polls which are coming out tonight suggest that voters in Michigan are just as — are less angry than voters in, say, Mississippi, and that they are more mainstream perhaps. How does that translate? Or does that translate? Does that match up with what you have been seeing?

KATHY GRAY: Well, there is some anger out there, too. There are a lot of people who are either unemployed or underemployed. There's a lot of wage stagnation there, so the message that Donald Trump is sending has kind of resonated with people here in Michigan.

But there are wide swathes of moderate voters, moderate Republican voters and independents in the state as well, and those are the folks who John Kasich is hoping to attract, and he's been here almost nonstop for the past week.

GWEN IFILL: Let's talk about Ohio Governor John Kasich, who is trying to make a dent in the Rust Belt somewhere.

Did he benefit at all from that really raucous debate last week where everybody was throwing mud and he was kind of trying to rise above?

KATHY GRAY: I think he did.

A lot of people that I have talked to, I have been at several of Kasich's event here's in Michigan, have said that they appreciated a person who is a little less blustery, a little less fiery. And he seemed to be kind of the grownup on the stage, and people in Michigan who were tuning in very much so to the debate in Detroit I think appreciated that.

GWEN IFILL: One of the things we have seen in other states as well is that a lot of early voters, people who cast their vote some time in the past, came out for Trump, but, in the end, the turn was toward other candidates, even though not enough to overtake the front-runner.

How has early voting been in Michigan?

KATHY GRAY: It's been huge.

I just talked with the secretary of state's office late this afternoon, and the absentee ballots are up by — it's going close to 200,000 more than in 2012. Early voting, we had a poll last week, shows that the early voting is benefiting both Donald Trump and Hillary Clinton to a very large degree.

So the voter turnout on today is going to be key for the rest of the candidates.

GWEN IFILL: You mentioned Hillary Clinton. It allows me to move to the Democrats. There's been a lot of discussion about the situation in Flint, the poisoned, lead-taunted water in Flint, Michigan. And of course, there was a Democratic debate there recently.

How has that issue resonated, either on the Democratic side or on the Republican side?

KATHY GRAY: It's really resonated on the Democratic side. Hillary Clinton was the first one in to call — talk about it. She talked about it first at a debate way back in January.

She came to — she came to Flint in early February. She's been pounding on it relentlessly for the past couple of months. Bernie Sanders has also been to Flint. He came and did a town hall meeting in Flint. And it is really resonating.

It's kind of a narrative of how inner cities, inner urban cities have been kind of left behind by Republican policies, and so they're definitely playing big on that.

Flint also has a very large African-American population, so both of the candidates have been trying to attract those voters as well.

GWEN IFILL: Let me try two numbers on you. This is on the Republican side, delegate — adding up the delegates at stake. Fifteen percent is one number. Fifty percent is the other.

Explain how that's going to play out.

KATHY GRAY: Well, on the Republican side, we have 59 delegates at stake. You have to get at least 15 percent of the vote to get that. So there's a very real possibility that one of the Republicans won't meet that threshold.

On the Democratic side, you have to get — it's a winner — it's a not a winner-take-all. It's proportional. There's 130 delegates at stake, another 17 superdelegates. Those 130 are divided up proportionately. And we expect that both Clinton and Sanders will get some delegates out of Michigan.

GWEN IFILL: OK. Kathy Gray of The Detroit Free Press, thank you. Long night ahead.

KATHY GRAY: Thank you.

GWEN IFILL: Get the latest on the election online. Check in with our results page all night. That's at PBS.org/NewsHour.