Donald Trump faces latest test as Michigan prepares to vote

Mississippi, Idaho and Hawaii are also holding Republican contests, while Democrats Hillary Clinton and Bernie Sanders face off in Michigan and Mississippi.

Squeezed between last week's high-profile Super Tuesday contests and high-stakes primaries next week in Florida and Ohio, today's contests are unlikely to dramatically reshape either party's primaries.

But with 150 Republican and 179 Democratic delegates to the parties' national nominating conventions at stake, the races offer an opportunity for front-runners to pad leads or for rivals to catch up.

While Mr Trump has stunned Republicans with his broad appeal, he has forged a particularly strong connection with white working-class voters by emphasising his opposition to international trade deals and support for building a wall along the US-Mexico border.

With an eye on the general election, he has argued he could put Midwestern Democratic-leaning industrial states such as Michigan and Wisconsin in play for Republicans.

Mr Trump is facing competition from Ohio governor John Kasich, who has failed to win a single primary so far but hopes Michigan can give him a boost heading into his home state's winner-take-all the delegates contest on March 15.

"It's not just the whole country that's watching Michigan - now the world's beginning to watch," Mr Kasich said during a campaign stop in the state.

"You can help me send a message about positive, about vision, about hope, about putting us together."

Unless Mr Kasich and Florida senator Marco Rubio can win in their home states next week, the Republican primary campaign is set to become a two-person race between Mr Trump and Ted Cruz.

Texas senator Mr Cruz, an uncompromising conservative who has publicly criticised party leaders, is sticking close to Mr Trump in the delegate count.

With six states in his win column, he is arguing he's the only candidate standing between the brash billionaire and the Republican nomination.

During a stop at a catfish restaurant in Mississippi, Mr Cruz said the current vacancy on the Supreme Court meant Republicans could not take a chance on Mr Trump.

"He's been supporting left-wing politicians for 40 years," Mr Cruz said.

More mainstream Republicans have cast both Mr Trump and Mr Cruz as unelectable in a November face-off with the Democratic nominee.

But they are quickly running out of easy options to stop their momentum and are increasingly weighing long-shot ideas such as a contested convention or rallying around a yet-to-be-determined third-party candidate.

Mrs Clinton, meanwhile, appears to be on a steady path to the Democratic nomination.

She has steadily grown her lead over Mr Sanders, who has struggled to broaden his appeal beyond a loyal following of younger voters and liberals.

Her surge led former New York mayor Michael Bloomberg to announce yesterday that he would not run for president as an independent candidate - a move that would have rocked this year's already extraordinarily unpredictable presidential campaign.

Mr Bloomberg, who had spent months mulling a third-party run, made his decision official through an editorial posted by the Bloomberg View, writing that he believes his candidacy would probably lead to the election of Mr Trump or Mr Cruz.

"That is not a risk I can take in good conscience," the 74-year-old billionaire wrote.

Those close to the process said Mr Bloomberg had believed the dominance of Mr Trump among Republicans and the rise of Mr Sanders amid Democrats had opened a centrist lane for a non-ideological, pragmatic campaign.

But Bloomberg aides say that path is now blocked with Mrs Clinton emerging as the likely Democratic nominee.

Today's results will offer clues about whether MrSanders is making any inroads in the overwhelming support that Mrs Clinton has enjoyed from black voters.

Trying to make a stand in Michigan, Mr Sanders has accused her of being disingenuous when she asserted that he opposed the industry bailout that rescued car makers General Motors and Chrysler during the Great Recession.

The 2008-2009 bailout by presidents George Bush and Barack Obama remains popular in Michigan, the home of the US car industry, and has been credited with preserving the Midwest's manufacturing base.

Heading into Tuesday's contests, Mrs Clinton had accumulated 1,130 delegates and Mr Sanders 499, including superdelegates. Democrats need 2,383 delegates to win the nomination.

Among Republicans, Mr Trump leads with 384 delegates, followed by Mr Cruz with 300, Mr Rubio with 151 delegates and Mr Kasich with 37.

Winning the Republican nomination requires 1,237 delegates.