In the Democratic primary, Hillary Clinton won but a dispute over the date of the vote led the national party to strip the state of its delegates to the nominating convention later this year, in effect rendering Tuesday's vote of no consequence.
Instead, Clinton and rivals Barack Obama and John Edwards were set to square off in an evening debate in Nevada, where the party's next contest is on Saturday.
Clinton court challenge
Clinton's supporters went to court in Nevada seeking to a change in ground rules for Saturday's caucuses or party meetings to indicate presidential preferences.
Democratic voters in Michigan did cast ballots on Tuesday but they will have no official bearing on the selection of the party's nominee.
The Democratic national committee punished the party's state body for defying it by scheduling the primary before "Super Tuesday" on February 5.
Michigan Democrats were stripped of the 156 delegates who would have represented the state during the party's national convention in August, when the nominee will be named.
Jennifer Granholm, Michigan's governor and a Democrat, defended the state party's decision, saying its voice would have been drowned out if it had been held along with about 20 other states on February 5 or on a later date.
Democratic candidates agreed to boycott the state and Barack Obama and John Edwards do not even appear on the ballots.
Michigan is an "open" primary, so Democratic voters may choose between Hillary Clinton and longshots Dennis Kucinich and Mike Gravel, or they may cross over to the Republican side and vote in their opponents' race.
The Democrats also stripped Florida of its 210 delegates for setting the state primary on January 29.
The Republican state parties in Michigan and Florida also disobeyed their national committee, but had only half of their delegates taken away.
Their objective was to prevent several caucuses along the Las Vegas strip of hotels and casinos, where thousands of Culinary Workers Union employees - many of them Hispanic or black – work.
The rules were approved in March, when the Clinton was the overwhelming national frontrunner in the race, but the union voted to endorse Obama last week.
The courts also figured in an evening debate in Nevada, where a state judge said Dennis Kucinich, a longshot candidate, must have a place on the stage.
MSNBC, with plans to televise the debate on cable television, had decided to exclude Kucinich after his poor performances in the Iowa caucuses and New Hampshire primary.
Kucinich said he has advocated positions his opponents have not on issues such as the Iraq war, trade and healthcare, "so my being there creates a debate".
The economy topped the agenda in Michigan, home of a struggling US car industry and the state with the highest unemployment rate in the US at 7.4 per cent, nearly 3 points above the national average.
McCain had promised lower taxes and reduced government spending would make the United States more competitive globally and create new jobs in the auto industry, but he said those jobs that had been lost would not be coming back.
Romney had accused McCain of being too pessimistic and after his projected win on Tuesday, said it was "a victory of optimism over Washington-style pessimism".
He had stressed his experience running the Salt Lake City Olympics as well as a venture capitalist and said he would restore Detroit's lost power by lifting the regulatory burden on companies and boosting research to generate new jobs.
Romney grew up in Michigan in a well known political family. His father was a senior car industry executive and a popular governor in the 1960s.