Washington: US voters cast ballots in White House primaries in Michigan and Mississippi, with Republican frontrunner Donald Trump and Democrat Hillary Clinton favoured to win their parties' latest test of strength.
Northern Michigan, the first big industrial state to weigh in in the presidential nomination race, and the southern Gulf state of Mississippi hold the biggest prizes of the day in terms of delegates and visibility.
But Republican voters also will have their say in a primary in Idaho and a caucus in Hawaii.
Clinton is seeking to further extend her lead in the delegate count over Bernie Sanders, the Vermont senator who has energised young voters with calls for greater economic equality and denunciations of what he sees as a corrupt US political system.
Republicans meanwhile were in an intensifying race that will soon tell whether Trump's march towards the nomination can be broken by rivals led by Senator Ted Cruz of Texas.
A new Washington Post poll of Republican-leaning registered voters shows Trump with 34 per cent support, compared with 25pc for Cruz, 18pc for Senator Marco Rubio and 13pc for Ohio Governor John Kasich.
That is a far tighter race than in January, when the Post showed Trump up 16 points against Cruz and 26 against Rubio.
But Trump has tightened his grip on the lead, winning 12 out of 20 state contests so far, in regions as varied as the industrial northeast and the deep south bible belt.
Cruz, the 45-year-old champion of the religious right, is nipping at the billionaire real estate mogul's heels.
Cruz has done well in delegate-rich Texas and nearby states, pulling ahead of the two other remaining Republican candidates, Rubio and Kasich.
Rubio, 44, has the backing of the mainstream Republican anti-Trump camp – which sees Cruz as too uncompromising to unite the Republican Party – but he has underperformed and trails in third place.
Next week's primary in Rubio's home state of Florida, a winner-take-all contest with 99 delegates at stake, is widely seen as a must-win if he is to remain a viable contender.
In an attempt to counter the frontrunner's advance, 2012 Republican nominee Mitt Romney has made anti-Trump robocalls rolling out in the states.
"If we Republicans were to choose Donald Trump as our nominee, the prospects for a safe and prosperous future would be greatly diminished, and I'm convinced Donald Trump would lose to Hillary Clinton," Romney said in calls recorded for Rubio and Kasich.
Major states Florida, Illinois and Ohio vote on March 15 in a sequel to last week's "Super Tuesday."
Trump has announced the launch of his own Florida ad campaign against Rubio, portraying him as corrupt by invoking an old credit card controversy.
"Little Marco Rubio, you know he's a no-show in the US senate," Trump said during a campaign stop in Concord, North Carolina.
Meanwhile Cruz insisted that he is the best alternative to Trump.
"Of course Donald is upset and I will predict he will engage in more attacks," Cruz told reporters in North Carolina.
"They will be more personal and nasty and we will not respond in kind but focus on issues, substance and policy because that's what voters expect in the end."
A total of 150 Republican delegates are up for grabs out of 1,237 needed to win the party's nomination. As of Monday Trump has 384 delegates, compared with 300 for Cruz, 151 for Rubio and 37 for Kasich.
Trump leads in Michigan with 36pc compared to 23pc for Cruz, according to a recent Monmouth University poll. But Kasich is eyeing a surprise, after actively campaigning in Michigan and receiving the endorsement of actor and former California Governor Arnold Schwarzenegger.
For Democrats, Clinton is the favourite. At a rally in Detroit a day earlier, the former secretary of state pivoted towards the GOP.
"The sooner I could become your nominee the more I could begin to turn our attention to the Republicans," she said.
Clinton has amassed 1,130 delegates, compared to 499 for Sanders, thanks in part to the hundreds of so-called "superdelegates" – elected officials and party operatives with a vote at the national convention in July – who have committed to Clinton in addition to the delegates won in primary contests so far.
A candidate needs 2,382 delegates to win the Democratic nomination.