After more than two weeks of quiet on the primary calendar, although certainly not on the campaign trail, the big day has arrived. Michigan and Arizona will hold their much-anticipated primaries throughout the course of the day, with the polls closing at 9 p.m. ET in both states.

Both states boast a relatively large number of registered voters. There are 3,183,327 registered voters in Arizona. The state counts 1,118,938 registered Republicans, about 35 percent of the registered voter population. Arizona's Republican primary is only open to registered Republicans. Voters wishing to participate were required to register with the party by Jan. 30.

Turnout in 2008 was high in the Republican primary in Arizona; almost 52 percent of registered Republicans cast their votes. Of course, part of the reason for the high turnout might be attributed to Arizona Sen. John McCain's being on the ballot.

Michigan has 7,279,629 registered voters. Voters do not register by party in Michigan, meaning that anyone can participate in the primary regardless of political leanings. Those who wish to participate were required to register by Jan. 30. Almost 21 percent of the registered voting population turned out to vote in the state's primary in 2008.

Neither state holds a particularly large delegate prize after both of the states lost half of their delegates as a punishment for scheduling their primaries ahead of March 1. Arizona has 29 delegates up for grabs, Michigan has 30. Arizona's primary will be a winner-take-all contest, while Michigan will dole its delegates out proportionally.

Although Arizona will offer a larger amount of delegates to the ultimate winner, because they will not have to share them with any other candidates, the contest has not gained the same level of attention - from either the media or the GOP field - as Michigan.

This is partly because Mitt Romney appears to have a comfortable lead in the polls. Romney finished second in Arizona in 2008 - garnering 35 percent of the vote - but he was competing against Arizona's  senior senator, who had served the state for decades.

Many of the votes in Arizona's primary have already been cast. Early voting began Feb. 2 in the state and continues straight through to election day. As of Monday, February 27, 326,439 ballots had been cast. (It should be noted that the Green Party primary also takes place Feb. 28.)

Arizona is home to sizable Mormon communities in Coconino County and Navajo County, which are located in the northern-middle portion of the state, along the Utah border. Look for Romney to take a big victory in those areas.

The numbers are much closer in Michigan, Romney's birthplace, where his father once served as governor and auto-industry executive. Romney won a decisive victory in Michigan in 2008, with 39 percent of the vote: 9 points higher than John McCain, who received 30 percent. This time around, however, the numbers are much closer, according to recent polling.

Michigan is make it or break for Romney. Not only would a loss in the Great Lakes State represent his third loss in a state he previously carried in 2008 (Colorado and Minnesota being the first two states), more importantly, it would represent a defeat in a place where he's considered to have the home-field advantage.

Romney has continuously played up his self-identification as a "son of Detroit" and although from a delegate perspective, a victory in Arizona and a loss in Michigan would still likely result in Romney's increasing his lead over the rest of the GOP field in the delegate count, the loss would be a huge symbolic blow to the campaign.

The areas of the state to watch in Michigan are Wayne County (where Detroit is located) along with Ottawa County in the middle-western portion of the state. The affluent suburbs surrounding Detroit in Wayne County are generally Republican strongholds in the state, and Romney took 44 percent of the vote there in 2008. Strong turnout in this county will likely bode well for Romney Tuesday.

Ottawa County is traditionally a stronghold for Christian conservatives. John McCain carried the county over Barack Obama in the general election in 2008. George W. Bush won strong victories in the county in 2000 and 2004. The county has a large population of fundamentalist Christians. Strong turnout here will probably be a good sign for Santorum.

Arizona and Michigan are the last two major primaries before Super Tuesday, March 6. Washington state will hold its caucuses March 3, but Tuesday's winner will carry the momentum from his victory into the 10-state voting contest the following week.