Voters in the US state of Michigan are voting in the state's presidential primary elections.
Mitt Romney is battling for political survival in a primary that could either revive or sink his campaign.
As the results roll in from Michigan soon after the last polls close at 0100 GMT, Democratic White House contenders Barack Obama, Hillary Clinton and John Edwards are scheduled to meet in an evening debate in Nevada, which is the site of the party's next contest on Saturday.
Polls show Mr Romney, a former Massachusetts governor who was raised in Michigan in a famous political family, running neck-and-neck with Arizona Senator John McCain in a state where the ailing economy has moved to the top of the agenda.
A Reuters/C-SPAN/Zogby poll released early today gave Mr McCain a statistically insignificant 1-point edge, 27% to 26%, over Mr Romney. Former Arkansas Governor Mike Huckabee was third with 15%.
Mr Romney needs a breakthrough win in Michigan to keep his White House hopes alive after second-place finishes in Iowa and New Hampshire.
A win for John McCain after last week's New Hampshire victory would thrust him into the front-runner's role heading into southern contests in South Carolina and Florida.
Democrats are also holding a primary in Michigan but a row between the local and national Democratic organisers over the date of the primary has reduced the significance of that poll.
As a result, Barack Obama and John Edwards have kept their names off the ballot.
While Hillary Clinton and a few minor candidates are on the ballot, Democrats can also vote 'uncommitted', which will probably mean delegates will be assigned at the Democratic convention in August.
The next big Republican contest will be the South Carolina primary this weekend.
