UPDATE: Mitt Romney won both the Arizona and Michigan primaries. Click here for analysis.

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The results of Michigan's primary Tuesday night could be a game-changer in the Republican presidential race, where Rick Santorum's surging poll numbers threaten Mitt Romney's chances in his home state.

Michigan is an important win for Romney, a former Massachusetts governor. If he loses to Santorum, a former Pennsylvania senator, it would be a major blow to any front-runner status Romney hoped to cling on to. It would signal that an already long primary race will drag even longer, at a time when most parties have rallied around a single candidate in other presidential elections.

The race isn't just affected by who wins, but by how much. Romney needs to beat Santorum by a significant amount--about five percent--to bury Santorum's momentum. As long as Santorum does better than what is expected--even if he loses to Romney but comes close--it would be positive news for his campaign.

Whatever happens, Michigan (and Arizona) will set the tone for the next week until Super Tuesday, when 10 states will hold their primaries. It will be interesting to see if the GOP settles on Romney, Santorum, or someone new they hope will jump into the race. If the latter is the case, the party is worried about the possibility of a brokered convention.

According to a Public Policy Polling survey from Feb. 26, Santorum is leading Romney by 1 percent, 38 to 37 percent respectively. Texas congressman Ron Paul follows with 14 percent, and Newt Gingrich, former speaker of the House, is in last with 9 percent.

Another poll by Rassmussen, a Republican-leaning company, has Romney leading 38 percent to Santroum's 36 percent.

Below is everything you need to know about following the race tonight:

(Click here for more information about the Arizona primary.)

What's at Stake?

Michigan has 30 delegates up for grabs, which are distributed proportionately to the candidates.

When Do the Results Come in?

Polls begin closing in Michigan at 8 p.m. local time (ET). The results will then trickle in during the next few hours.

Where to Watch Live Online

CNN will be streaming coverage online live beginning at 6 p.m. ET at live.cnn.com with a special edition of John King, USA.

Fox News will be reporting the results on their website at FoxNews.com Live, beginning at 8:45 p.m. ET.

Politico will also be live streaming coverage hosted by Jim VandeHei and Mike Allen, beginning at 6:30 p.m. ET on politico.com/livestream.

The Huffington Post's live blog includes tweets, aggregated and original content.

For raw coverage of results and the candidates speeches, tune into C-SPAN.

Where to Follow

For real-time updates on Twitter, search for the hash tag #MIprimary. We also recommend this Twitter list of politicos, candidates and journalists who will be watching the contests very closely tonight: https://twitter.com/#!/sara_dover/politics

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