Deepening their dominance in the South, Democrat Hillary Clinton and Republican Donald Trump swept to victory Tuesday in Mississippi's presidential primaries. Voting was still underway in Michigan, the night's biggest prize.
Ms. Clinton and Mr. Trump hoped Tuesday's contests would pad their delegate leads and move them closer toward a general election face-off. Democrat Bernie Sanders sought to slow Clinton in Michigan, while a trio of GOP candidates desperately tried to block Trump's path the nomination.
Trump has faced a barrage of criticism from rival candidates and outside groups who fear he would deeply damage Republicans' chances of winning in November. His recent losses to Ted Cruz, the Texas senator, have raised questions about his durability and given fresh hope to other competitors.
Ohio Gov. John Kasich campaigned furiously in Michigan in recent days. He's yet to win a primary but hoped a good showing in Michigan would give him a boost heading into next week's crucial contest in his home state.
Speaking to a crowd in Lansing, Governor Kasich said a strong showing in Michigan would show the country "that it's a new day in this presidential campaign."
To that end, Kasich and Florida Sen. Marco Rubio used recorded phone calls from Mitt Romney to appeal to voters as they headed to the polls. Mr. Romney, the 2012 GOP nominee, has not endorsed a candidate but has vowed to help challengers to Trump, who he says would be dangerous for the country.
Republicans were also holding contests Tuesday in Hawaii and Idaho. GOP candidates were fighting for 150 delegates, while 179 Democratic delegates were at stake in the party's two primaries.
The economy ranked high on the list of concerns for voters heading to the polls in Michigan and Mississippi. At least 8 in 10 voters in each party's primary said they were worried about where the American economy is heading, according to exit polls conducted by Edison Research for The Associated Press and television networks.
Among Democrats, 8 in 10 voters in both states said the country's economic system benefits the wealthy, not all Americans.
Senator Sanders has sought to tap into that concern, energizing young people and white, blue-collar voters with his calls for breaking up Wall Street banks and making tuition free at public colleges and universities. Michigan, with big college towns and a sizable population of working-class voters, should be a good fit for him. But Clinton has led in polling.
Tuesday's contests are a prelude to next week's high-stakes primaries in Florida and Ohio. Like Kasich, Senator Rubio must win his home state in order to remain a viable contender.
"It has to happen here, and it has to happen now," Rubio told supporters Tuesday during a rally in Sarasota.
Rubio has received endorsements from a steady stream of senators, governors and other high-profile Republican officials. But his backing from voters has lagged, and he entered Tuesday's contests with just a pair of victories in the Minnesota caucuses and Saturday's Puerto Rico primary.
If Rubio and Kasich can't win at home, the GOP primary appears set to become a two-person race between Trump and Senator Cruz. The Texas senator is sticking close to Trump in the delegate count and with six states in his win column, he's arguing he's the only candidate standing between the brash billionaire and the GOP nomination.
During a campaign stop at a North Carolina church Tuesday, Cruz took on Trump for asking rally attendees to pledge their allegiance to him. He said the move strikes him as "profoundly wrong" and is something "kings and queens demand" of their subjects.
"I'm not here asking any of you to pledge your support of me," Cruz said, to thunderous applause and cheers. "I'm pledging my support of you."
Some mainstream Republicans have cast both Trump and Cruz as unelectable in a November face-off with the Democratic nominee. But they're quickly running out of options and are increasingly weighing long-shot ideas such as a contested convention or rallying around a yet-to-be-determined third-party candidate.
Heading into Tuesday, Trump led the Republican field with 384 delegates, followed by Cruz with 300, Rubio with 151 and Kasich with 37. Winning the GOP nomination requires 1,237 delegates.
Among Democrats, Clinton had accumulated 1,134 delegates and Sanders 502, including superdelegates. Democrats need 2,383 delegates to win the nomination.
Pace reported from Washington. Associated Press writer Emily Wagster in Jackson, Mississippi, Kathleen Ronayne in Monroe, Michigan, and Steve Peoples in Sarasota, Florida contributed to this report.