Voters in four US states head to the polls Tuesday to confirm or rein in Donald Trump's status as clear Republican frontrunner in primary contests that Democrat Hillary Clinton hopes will further solidify her lead over Bernie Sanders.
As the race for the White House heats up, the spotlight shifts to the Rust Belt state of Michigan and Mississippi, where both Republican and Democratic balloting will take place.
Republicans will vote in a primary in Idaho while Hawaii hosts a Republican caucus.
The latest round of voting comes amid a rapidly evolving situation in the Republican fight for the nomination.
With every passing week, billionaire real estate mogul Trump is seeing his lead reinforced. Having so far pocketed 12 out of 20 contests, the 69-year-old has the most delegates in hand and saw success in regions as different as the Northeast and South.
But nipping at his heels is Texas Senator Ted Cruz, 45-year-old champion of the religious right, who did well at home and in nearby states.
However, the Republican anti-Trump camp -- who has considered Cruz to be too uncompromising to unite the Republican party -- has been reluctant to support Cruz over Marco Rubio, 44, trailing in third place, whose survival test will be the primary next week in his homestate of Florida.
In an attempt to counter Trump's advance, campaign ads depicting him 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, who could be forced to throw in the towel if he does not do well on his home turf. The ad portrays Rubio as corrupt by invoking an old credit card matter.
"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.
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"We're now the only campaign that has beaten Donald Trump over and over and over again," Cruz, meanwhile, told reporters.
A total of 150 Republican delegates are up for grabs Tuesday out of 1,237 needed to win the party's nomination, compared to 166 out of 4,763 on the Democratic side.
Trump is leading in Michigan with 36 percent compared to 23 percent for Cruz, according to a recent Monmouth University poll, although rival John Kasich, the governor of neighboring Ohio, is hoping for a surprise in his favor after being active on the ground.
There are no recent polls in Mississippi but last month Trump largely dominated his rivals there.
For Democrats, Clinton is the favorite in both states. That is especially true in Mississippi where African American voters represent an important voting bloc. African Americans have voted by more than 80 percent for Clinton in other states of the South.
Michigan and the city of Detroit symbolize the heart of the US auto industry, and Clinton has accused Sanders of having voted against a plan to save the sector in 2008/2009.
"I voted for the auto bailout and he voted against it," she repeated Monday while visiting a small software firm in Grand Rapids, Michigan.
Sanders did vote for it but separately voted against funds whose main purpose was to assist financial institutions but which also ended up financing the auto industry bailout.
The Vermont senator, for his part, sought to stress to voters that Clinton has supported free-trade agreements that he said has cost millions of jobs.