Update, 10:24 p.m.: The Associated Press and NBC have declared Mitt Romney the winner of the Michigan primary. Romney performed well in southeastern Michigan, particularly in Macomb and Oakland counties. With 76 percent of the precincts reporting in Macomb, Romney had 43 percent of the vote compared to 34.8 percent for Santorum.
Update, 9:58 p.m.: Mitt Romney has won the Republican primary in Arizona, according to the Huffington Post. The victory garners him 29 new delegates on his quest to gain his party's nomination for president. Meanwhile, it's too close to call in Michigan, according to the Detroit Free Press. Romney and Rick Santorum are in a dead heat, with about 38% of precincts reporting. Both men have made several stops in Michigan in recent weeks.
Update, 8:30 p.m.: Supporters of Mitt Romney are gathering at the Suburban Collection in Novi, where state Rep. Chuck Moss (R-Birmingham) was spotted outside the ballroom.
Moss, donning a lapel button from one of George Romney's gubernatorial campaigns in the 1960s, said he fully supports the current GOP presidential hopeful. He said he was shocked to get a robo call today from opponent Rick Santorum's camp urging Democrats to support him in today's voting.
"I really couldn't believe it, but he copped to it. I guess he's got to try," Moss said.
As for the campaign button, Moss said he received it from his father-in-law, who was a big Romney supporter and was part of Mitt Romney's father's initial campaign from 1962.
Update, 7:45 p.m.: Reports of low voter turnout are coming in from throughout southeast Michigan.
Ann Bollin is estimating a 17-18 percent turnout compared to the 25 percent she predicted last week, while reported "stretches of up to 45 minutes with no one walking through the door."
Joe Munem said precincts were averaging 4.5 percent turnout at 5 p.m.
Resident Robert Schwartz, 62, who did vote this evening, said he wasn't surprised by the low turnout. "I think people are probably put off by the candidates," Schwartz said. "They've been running a very negative campaign and I think it turns people off."
Update, 4:45 p.m.: Between Mitt Romney's charges that Rick Santorum is attempting to "kidnap" the Republican presidential nomination process, comments on social media and voter interviews, it is clear some Democrats are joining Republicans at the polls today to weigh in on the GOP race.
Santorum’s campaign is making automated phone calls to Michigan Democrats to urge them to vote for their candidate, citing Romney’s opposition to the $82 billion federal automotive bailout, which Santorum also opposed.
Romney told reporters the calls are the “dirty tricks of a desperate campaign.”
“Republicans have to recognize there’s a real effort to kidnap our primary process,” Romney said Tuesday morning in Livonia.
Margaret Weiss, a registered Democrat from Ferndale, told The Detroit News she cast her vote for U.S. Rep. Ron Paul of Texas.
"I hate the other two," said Weiss, 64, referring to former Massachusetts Gov. Mitt Romney and former Pennsylvania Sen. Rick Santorum. "I like what Ron Paul stands for; he is more for the people and less for the money."
It was unclear the extent of the Democrats' efforts or whether they would be enough to dramatically change the day's outcome.
Even among Republicans, there was some voting against rather than voting for a candidate.
On the north side of St. Clair Shores, Sally Rosberg cast her vote for Santorum at .
"Every time I hear Romney say 'Let Detroit go bankrupt,' it makes me furious," Rosberg said. "I don't know if will (vote for Santorum) in the general election."
Thomas Goad, a long-time General Motors employee and Romney supporter, voted in Birmingham.
"I've known him the longest and I trust him the most," Goad said, adding that he "was not worried" about Romney's stance on the automotive industry industry. "I never considered anyone else. Rick Santorum for some reason he just turns me off with his speeches."
Michigan primary a 'must win' for Romney
11:30 a.m.: While Michigan voters are today to vote for a Republican candidate in the 2012 presidential primary, several analysts and major media outlets are saying the results of this race could influence the entire campaign.
The LA Times is among those calling today's primary the first "must-win" for Mitt Romney, the former governor of Massachusetts and a Michigan native. He's planning to go head-to-head with Rick Santorum, who got a slow start in the race but is slowly becoming the favorite among more conservative members of the Republican Party.
At stake in Michigan today are 30 delegates, half the original 59, because the state chose to have its primary earlier than national party rules allow.
Two delegates will be awarded to the winning candidate in each of Michigan's 14 congressional districts, for a total of 28 delegates, and the winner of the state overall will get two additional delegates.
Arizona is also holding a primary today, where 29 delegates are up for grabs and Romney is favored.
It's not just delegates the candidates are after. They're also looking for momentum, which, as the Washington Post points out, will propel the winner into strong standing just in time for Super Tuesday on March 6, when 10 states hold primaries.
Santorum and Romney are staking out their claims on either side of the state, with Santorum spending today in conservative west Michigan, starting making not one, but two breakfast appearances, one each in Grand Rapids and Kentwood.
Romney also kicked off primary day with breakfast at Senate Coney Island in Livonia before moving on to a round table discussion at Bizdom U in Detroit.
Turnout at the polls has been fairly light so far today. State election officials expect 15 percent to 20 percent of registered voters to cast ballots.
Follow Patch for updated news on the presidential primary all day long.