Republican Donald Trump and Democrat Hillary Clinton will fight for support to be nominated for the presidency from their respective parties in four states on Tuesday, but the focus will be mainly on Michigan - the first big industrial state to weigh in.
Polls show both nominees comfortably ahead in the Michigan vote, which will indicate how they will play in important primaries in the Midwest still to come.
Republicans will also vote in a primary in Idaho in addition to the primary in Michigan, while Hawaii will host a Republican caucus.
The latest round of voting comes amid a rapidly developing situation in the Republican fight for the nomination.
Trump has so far won 12 out of 20 Republican primaires and has 384 delegates, while his closest rival Ted Cruz has 300. He needs to reach 1,237 delegates to win the party's nomination for the November 8 presidential election.
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, who is trailing in third place with 151 delegates.
"Trump has been holding strong and no one seems to be catching up," said Republican pollster Steve Mitchell.
Trump is leading in Michigan with 36 percent compared to 23 percent for Cruz, according to a separate Monmouth University poll. There are no recent polls in Mississippi but last month Trump largely dominated his rivals there.
Clinton, on the other side, has 672 delegates so far, while her avowedly socialist rival Bernie Sanders has 477. She needs 2,383 delegates to be nominated by the Democratic Party.
African American voters represent an important source of support for Clinton and have voted by more than 80 percent in her favour in states in the South.
Michigan symbolises 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 Michigan.
Sanders did vote for the bailout, 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, has said that Clinton has supported free-trade agreements that have cost Americans millions of jobs.