Republican front-runner Donald Trump racked up primary wins in the big prize of Michigan and in Mississippi on Tuesday, brushing off a week of blistering attacks from the party's establishment and expanding his lead in the White House nominating race.
In the Democratic contest, Bernie Sanders stunned front-runner Hillary Clinton in a narrow Michigan primary upset, giving his upstart campaign new energy. Clinton won in Mississippi, but Sanders' victory is seen as likely to ensure a prolonged fight to pick a candidate for November's general election.
Trump's convincing win in Michigan restored his outsider campaign's momentum and increased the pressure on the party's anti-Trump forces to find a way to stop the brash billionaire's march to the nomination ahead of several key contests next week.
The 69-year-old New Yorker built his victories in Michigan, in the heart of the industrial Midwest, and Mississippi in the Deep South with broad appeal across many demographics. He won evangelical Christians, Republicans, independents, those who wanted an outsider and those who said they were angry about how the federal government is working, according to exit polls.
At a news conference afterward, Trump said he was drawing new voters to the Republican Party and the establishment figures who are resisting his campaign should save their money and focus on beating the Democrats in November.
"I hope Republicans will embrace it," Trump said of his campaign. "We have something going that is so good, we should grab each other and unify the party."
The results were a setback for rival John Kasich, governor of Ohio, who had hoped to pull off a surprise win in neighbouring Michigan, and Marco Rubio, a US senator from Florida who has become the establishment favourite but lagged badly in both Michigan and Mississippi and appeared unlikely to win delegates in either.
Trump said Rubio's recent attacks on him had backfired.
"Hostility works for some people; it doesn't work for everyone," the real estate magnate said at a news conference in Jupiter, Florida.
Ted Cruz, a US senator from Texas whose recent victories have positioned him as the prime alternative to the brash billionaire, won the party's primary in Idaho.
But Trump suggested his rivals had little hope going forward, and took particular aim at Cruz.
'A HARD TIME'
"Ted is going to have a hard time," Trump said of Cruz. "He rarely beats me."
Trump continues to enjoy a wide lead nationally in the Republican race, although Cruz has been climbing over the past week. Among those who identify as Republicans, Trump has settled in at about 40 percent support, according to a five-day rolling average ending on Tuesday in the Reuters/Ipsos poll.
Cruz at 23 percent and Kasich at 11 percent have been on the rise, largely at Rubio's expense.
The Michigan victory sets Trump up for a potentially decisive day of voting a week from Tuesday. On March 15, Ohio, Florida, Illinois, Missouri and North Carolina - like Michigan, states rich in the delegates who will select their party's nominee at July's Republican National Convention - cast ballots.
The Republican contests in Florida and Ohio award all the state's delegates to the winner. If Trump could sweep those two states and pile up delegates elsewhere next week, it could knock home-state favorites Rubio and Kasich out of the race and make it tough for Cruz to catch him.
"The biggest takeaway is that the Republican establishment is in its death throes. The only remaining candidates are 100 percent anti-establishment," said Mark Meckler, an early Tea Party movement founder.
Republicans were also voting on Tuesday in caucuses in Hawaii.
Many mainstream Republicans have been offended by Trump's statements on Muslims, immigrants and women and alarmed by his threats to international trade deals. Trump said on Tuesday he has not assembled a foreign policy team, despite having said he would have one in place by February, and dismissed criticism his statements would be harmful to U.S. interests.
Anti-Trump Super PACS have spent millions of dollars on advertisements designed to attack Trump's character in Florida, a state Rubio calls home and Trump calls a second home.
But Trump's relentless anti-free trade rhetoric and promise to slap taxes on cars and parts shipped in from Mexico resonated in Michigan, which has lost tens of thousands of manufacturing and auto industry jobs.
"The solid victory in Michigan is based on his populist message about bringing industry back to this country," Meckler said.
In the Democratic race, Sanders told reporters in Florida that the results in Michigan were a repudiation of the opinion polls and pundits who had written off his chances in the state. Polls had shown Clinton with a double-digit lead going into the primary.
The US senator from Vermont, a democratic socialist, said the win showed his political revolution was "strong in every part of the country. Frankly, we believe our strongest areas are yet to come."
Clinton's campaign signalled ahead of Michigan that the race could be tight. Clinton, her husband, former President Bill Clinton, and daughter Chelsea Clinton all campaigned in the state over the past few days trying to garner last-minute votes.