The two presidential front-runners are both eyeing Super Tuesday 2 as a chance to build on their momentum and pad their leads as they look to put the races away by the end of the month.
Hillary Clinton will try to set the tone for the entire Great Lakes region by holding off Bernie Sanders in Michigan, and she is expected to claim another Southern victory in Mississippi.
And Donald Trump will try run the table, with four Republican contests on tap. He leads in Michigan, and has done well in Southern states like Mississippi. Idaho and Hawaii vote Tuesday as well, a chance to pick up more delegates.
Here's what to watch in the day's elections:
Can Trump break away?
Time is running out for Trump's opponents to turn things around before the winner-takes-all states start to vote and the GOP front-runner racks up enough delegates to lock the nomination up before the convention.
Trump and Texas Sen. Ted Cruz have broken away from Florida Sen. Marco Rubio in the delegate count -- and Tuesday should help them build on their advantage.
Trump leads the polls in Michigan, the crown jewel of Tuesday's contests.
Mississippi, meanwhile, will test whether Cruz's surge in Louisiana -- he closed a massive polling gap and finished within four points on Saturday -- expands to other Southern states, or Trump can finish running the table in the Deep South. Idaho could be better ground for Cruz, who has spent more time in the state than other candidates.
All three states, though, pose problems for Rubio.
Mississippi and Michigan each require candidates to reach 15 percent to accumulate delegates. Idaho requires candidates to top 20 percent. The stronger Trump and Cruz run, the tougher it becomes for Rubio to make any gains at all.
What about the late deciders?
Rubio has a history of doing well with "late deciders" -- voters who make up their minds in the last week. He picked up 39 percent of those voters in Virginia, for example, according to exit polls.
But those voters, polls have shown, typically make up less than one-third of a state's electorate.
In Michigan, a Monmouth University poll showed 23 percent of voters were undecided or only had a slight preference in the days leading up to the primary.
And supporters of Trump, polls have shown, tend to be more locked-in on their candidate than supporters of Rubio.
What Tuesday's contests could demonstrate is whether there's enough late movement to help Trump's rivals -- particularly since the week leading up to the debate featured the most explosive Republican debate yet.
The Democratic bellwether
After Sunday night's Democratic debate, Sanders campaign manager Jeff Weaver raised eyebrows when he referred to Clinton as a "regional candidate" who struggles to win outside the South.
For Clinton, Michigan will put that claim to the test.
She's increasingly eager to turn her attention to the general election -- and to do that, Clinton needs to turn the tide with white voters, and particularly run up her advantage among women. She has performed better in those demographics since Sanders won in New Hampshire just over a month ago, and Michigan is a good bellwether for Clinton. It's a heavily-populated, diverse state that Democrats absolutely have to have in order to win the general election in November.
Sunday night in Flint, Mich., Clinton dropped a bomb on Sanders, bringing up his 2008 vote against the auto bailout bill, setting up a line of attack she will likely use in other midwestern industrial states down the line.
"The money was there and had to be released in order to save the auto industry and 4 million jobs and to begin the restructuring. I voted to save the auto industry. He voted against the money that ended up saving the auto industry. I think that is a pretty big difference," she said at the CNN debate.
"Given the terrible pressures that the auto industry was under and that the middle class of this state and Ohio and Indiana and Illinois and Wisconsin and Missouri and other places in the Midwest were facing, I think it was the right decision to heed what President-elect Obama asked us to do," she added. "You were either for saving the auto industry or against it. I voted to save the auto industry and I'm very glad that I did."
A victory in Michigan could set Clinton up for a huge March 15 -- when Florida, Ohio, Illinois, North Carolina and Missouri vote. If Sanders can't win Michigan, he might not be able to win any of those states. A Clinton loss, though, means Missouri, Ohio and potentially Illinois could be in play.
Ohio Gov. John Kasich has actually jumped ahead of Rubio for third place in Michigan, and is rising quickly, a Monmouth University poll out Monday showed. He appears to have worn well in last week's Republican presidential debate, when he stayed out of the Trump-Rubio-Cruz scrum.
So imagine this scenario: Kasich beats Rubio in Michigan. Then, on March 15, Kasich wins his 66-delegate, winner-take-all home state of Ohio, and Rubio loses his 99-delegate, winner-take-all home state of Florida.
Suddenly, Kasich would become the leading moderate, establishment-type Republican in the race -- and Rubio would lack a path forward.
There are a lot of "ifs" for that to happen. But for Kasich to stand any chance of turning what's been a smaller-scale campaign that's been much choosier about where he tries to compete into one with a real shot at quickly racking up delegates, Michigan is where it has to start.
Winning Ohio could help Kasich play a small role in denying Trump the delegates he needs to win the GOP nomination outright.
But to have a shot at the nomination himself, Kasich has to win the Midwest -- states like Michigan, Illinois, Indiana and Wisconsin, which have similarly-styled Republican governors and are often general election battlegrounds.