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Donald Trump has won the Michigan Republican presidential primary by a large margin, with rival Nikki Haley coming in second, while US President Joe Biden triumphed in the Democrat vote in the state that is home to a large number of Arab-Americans.
Many Democrats voted "uncommitted", in protest against the US's role in the Gaza war.
Protest votes exceeded organisers' expectations, with more than 90,000 votes after 82 per cent of the Democratic ballots had been counted, figures by Edison Research showed.
That was nine times more than the target of 10,000 votes.
"Our movement emerged victorious tonight and massively surpassed our expectations," the Listen to Michigan campaign, which urged people to vote uncommitted, said.
"Tens of thousands of Michigan Democrats, many of whom who voted for Biden in 2020, are uncommitted to his re-election due to the war in Gaza."
Although Mr Biden and Mr 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.
For many Arab-Americans a vote for Mr Trump is unlikely - after his widely criticised ban on the entry of citizens from Muslim-majority countries, the move of the US embassy in Israel from Tel Aviv to Jerusalem and closing down the Palestine Liberation Organisation offices in Washington.
Mr Trump has consistently defeated Ms Haley in every primary race, but she had earlier vowed to press on after defeat in South Carolina, before Michigan and "Super Tuesday" on March 5.
Following his win on Tuesday, Mr Biden thanked his supporters making no mention of Gaza or the "uncommitted" votes while attacking his rival Mr Trump.
"Donald Trump is threatening to drag us even further into the past as he pursues revenge and retribution," Mr Biden said late on Tuesday.
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.
The state plays a decisive role in US presidential elections, which will be held on November 5 and so far appear to be another re-match between Mr Biden and Mr Trump.
It is a battleground state that could swing towards either party. In 2020, Mr Biden beat his rival by just 2.8 points.
