US voters cast ballots in White House primaries in Michigan and Mississippi Tuesday, with Republican frontrunner Donald Trump and Democrat Hillary Clinton favored to win their parties' latest tests of strength.
Republican voters also will have their say later in the day in a primary in Idaho and a caucus in Hawaii.
But the biggest prizes of the day in terms of delegates and visibility were in Michigan and Mississippi, where polls opened at 1200 GMT.
Clinton is seeking to further extend her lead in the delegate count over Bernie Sanders, the Vermont senator who has energized young voters with 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 toward the Republican nomination can be broken by rivals led by Texas Senator Ted Cruz.
With every passing week, the billionaire real estate mogul has reinforced his lead, winning 12 out of 20 contests so far, in regions as varied as the industrial northeast and the deep South bible belt.
But Cruz, the 45-year-old champion of the religious right, is nipping at his heels.
He has done well in delegate-rich Texas and nearby states, pulling ahead of the two other remaining Republican candidates, Senator Marco Rubio of Florida and Ohio Governor John Kasich.
Rubio, 44, has the backing of the 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 stay in the race.
In an attempt to counter the front-runner's advance, campaign ads depicting Trump as a charlatan have been financed by Republicans dismayed that a man who once flirted with the Democratic party could become the GOP flag bearer in November's presidential elections.
One such ad, to be shown in Florida, shows snippets of Trump using profane language while out on the campaign trail. Financed by the American Future Fund -- which has not revealed its donors -- the ad campaign has a budget of several million dollars, according to its spokesman Stuart Roy.
Other ads, created by the Club for Growth, target Republicans in Illinois.
Florida, Illinois and other states will vote on March 15 in a sequel to March 1'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 Monday during a campaign stop in Concord, North Carolina.
Meanwhile Cruz insisted that he is the best alternative to Trump.
"We're now the only campaign that has beaten Donald Trump over and over and over again," Cruz told reporters.
