It's the first contest... and the Iowa caucuses could mean the end of the line for some candidates.
"This shapes the storyline," says political analyst Susan Demas.
Everything's on the line for Donald Trump and Hillary Clinton, and if they win by huge margins, Demas says it could be a done deal.
"If we see Trump win in Iowa, we've seen he's had very strong poll numbers in Michigan," Demas said. "And Clinton has initially led the field in Michigan, if she wins in Iowa, I would be very surprised to see a Sanders win here."
But if it's close tonight, that's a game changer.
"These contests are full of surprises," Demas said. "If everything went the way that everyone thought it would, nobody would have to vote, we could just go with computer modeling."
The Michigan primary falls after 25 other states and territories.
With so many republicans on the list, Michigan could be more important down the line.
"That gives us a chance to influence the outcome of the presidential election," says political expert Dr. Matt Grossmann from MSU.
And he thinks the more Republicans that are still on the ballot by March, the bigger role Michigan will play.
"By the time we get to Michigan on March 8th, we'll still have a very important role in the process because we'll be the last one before there start to be more winner take all contests," Grossmann said.
As for the democrats, Demas thinks their front runner will be pretty set in stone by March 8th, but, she says anything can happen.