Republican frontrunner Donald Trump scored two major victories Tuesday in the states of Michigan and Mississippi, extending his lead in the US presidential nominations race ahead of crucial contests next week.
His Democratic counterpart Hillary Clinton also claimed victory in the southern Gulf state of Mississippi over her rival Bernie Sanders, thanks to a strong turnout by African-American voters.
"Thank you Mississippi!" Clinton and Trump posted in identical tweets.
But in the northern industrial state of Michigan, Clinton found herself narrowly trailing Sanders, the Vermont senator who has energized young voters with calls for greater economic equality and denunciations of what he sees as a corrupt US political system.
Clinton has now won 12 out of 21 contests, with Trump prevailing in 14 out of 22 races as the two inch closer to the tipping point in their respective nomination races.
US networks called the Gulf state of Mississippi for Clinton immediately after polls closed there, with exit polls reportedly showing very high participation among African-American voters.
Blacks comprised a stunning 69 per cent of the Democratic vote there, with an overwhelming 89 per cent of that demographic casting ballots for Clinton.
Mississippi Republican voters are also overwhelmingly evangelical, a group Trump has claimed to do well with.
Early results showed Clinton's rival, Senator Bernie Sanders of Vermont, leading by four percentage points in Michigan with 21 per cent of the vote counted, while Trump was ahead of Ohio Governor John Kasich by 12 points.
Senator Ted Cruz, who has emerged as the most viable alternative to Trump at least in terms of the all-important delegate count, was third. Marco Rubio, the senator whom mainstream Republicans rallied behind as the man to topple Trump, trailed in fourth.
"They didn't do so well tonight, folks," Trump said in a victory speech in Florida.
"Only one person did well tonight: Donald Trump."
In a somewhat bizarre scene, Trump spent several minutes hawking his products -- a steaks company, a winery, Trump vodka, even his Trump University -- which establishment critics had berated as examples of failed Trump businesses.
Republicans were voting Tuesday in two other states as well: a primary in Idaho and a caucus in the island state of Hawaii.
But the big prizes for both parties in terms of delegates and visibility unquestionably are Michigan and Mississippi -- two diverse states with different economies and demographic makeup.
By claiming both, Trump solidifies his claim that he has the broadest appeal among the Republican electorate as he marches toward the nomination.
But a new Washington Post poll of Republican-leaning registered voters shows Trump with 34 per cent support, compared with 25 per cent for Cruz, 18 per cent for Rubio and 13 percent for Kasich.
That is a 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 14 out of 22 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, Cruz has done well in delegate-rich Texas and nearby states and is nipping at the billionaire real estate mogul's heels.
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.
Trump 'would lose' to Clinton
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.
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 Tuesday 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.
Clinton was hoping Tuesday could be the day she pivots toward the GOP.
"The sooner I could become your nominee, the more I could begin to turn our attention to the Republicans," she said at a rally Monday in Detroit.
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.