Trump, Biden win Michigan primaries but Democrats mount Gaza protest vote

US President Joe Biden easily won the Democratic presidential primary in Michigan on Tuesday, Edison Research projected, but a protest vote by Democrats angry over his support for Israel’s war against Hamas in Gaza was growing as returns were counted.

Donald Trump won the state's Republican presidential primary by a large margin, strengthening his grip on the party’s White House nomination as Nikki Haley, his last remaining rival, came in a distant second.

Although Biden and Republican former President Trump had been expected to easily win their separate party primaries, the vote count for both was being closely watched for signs of wavering support.

In Michigan, home to a large Arab American constituency, Democratic voters had been urged to mark their primary ballots as "uncommitted" on Tuesday in protest at Biden's Gaza policy.

Early returns showed the number of "uncommitted" voters had exceeded 24,500 after 16 per cent of Democratic ballots were counted. That was more than double the 10,000 target for uncommitted voters that protest organisers had been hoping for.

Many in Michigan's Arab American community who backed Biden in 2020 are angry, along with some progressive Democrats, over Biden's support for Israel's offensive in Gaza.

"There's a lot of folks that are voting uncommitted who I think were not directly associated with the Arab American community," said Michigan pollster Bernie Porn. He said it remained to be seen how big of an issue the Middle East would be for Biden in November when the situation could look different.

Early returns showed Biden and Trump with solid overall leads. With 16 per cent of the estimated Democratic vote counted, Biden had 80 per cent  support, with "uncommitted" getting 15 per cent, and with 17 per cent of the estimated Republican vote counted, Trump had 66 per cent support to Haley's 29 per cent, according to Edison Research.

Michigan routinely offers an “uncommitted” option as a way of questioning whether a named candidate has the support of the party’s base. It could not be determined how many of those votes were protesting Biden's Gaza policy.

Michigan is expected to play a decisive role in the head-to-head November 5 US presidential election, a likely rematch between Biden and Trump.

It is a battleground state that could swing toward either party. Biden beat Trump in Michigan by just 2.8 percentage points in the 2020 election.