Republican front-runner Donald Trump has rolled to primary wins in the big prize of Michigan and in Mississippi, brushing off a week of blistering attacks from the party's establishment, while Democrat Bernie Sanders has stunned Hillary Clinton in a narrow Michigan primary upset.
Trump's convincing win in Michigan on Tuesday (local time) restored his outsider campaign's momentum and increased the pressure on the party's anti-Trump forces to find a way to stop his march to the nomination before several key contests next week.
Sanders has given his upstart campaign new momentum heading into bigger contests down the road. Clinton won the Mississippi primary.
Republicans were also voting in Idaho and Hawaii.
Trump 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 was working, according to exit polls.
At a news conference, Trump said he was drawing new voters to the Republican Party and the establishment figures who were 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 hoped to pull off a surprise win in neighbouring Michigan, and for Ted Cruz, a US senator from Texas who hoped to do well in Mississippi with its large bloc of evangelical voters.
Marco Rubio, a US senator from Florida who has been the establishment favourite since other mainstream candidates dropped out of the race, lagged badly in both states 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," Trump said at a news conference in Jupiter, Florida.
Trump suggested his rivals had little hope going forward and took aim at Cruz, who split four nominating contests on Saturday with Trump and positioned himself as the prime alternative to the brash New York billionaire in the race to be the party's candidate in the November 8 election.
The Michigan victory sets Trump up for a potentially decisive day of voting on March 15, when 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.
On the Democratic side, Sanders told reporters in Florida that the results in Michigan had been a repudiation of the opinion polls and the pundits who had written off his chances in the state.
Opinion polls had shown Clinton with a double-digit lead going into the primary.
The US senator from Vermont said it showed his political revolution was "strong in every part of the country. Frankly, we believe our strongest areas are yet to come".