Tuesday's primary results could propel Republican Donald Trump and Democrat Hillary Clinton to a nearly unstoppable path to their partys' presidential nominations. It could also leave the race even more jumbled than it already is. Here are some of the key counties and demographics that will help decide how Tuesday's big round of primaries play out:
Florida and Ohio: Trump's Chances for Very Important Wins
Florida and Ohio are very crucial to this Republican race. Because both states give all their delegates (99 in Florida, 66 in Ohio) to the winner of the primary, they will give one candidate a huge boost and provide nothing for the losers. Also, John Kasich and Marco Rubio have suggested they will leave the race if they don't win their home states, so the GOP contest could shrink in half after Tuesday.
If Trump wins both states, he is in a very strong position to get the 1,237 delegates needed for the GOP nomination.
Can he? Generally, the electorates of Florida and Ohio are favorable to him. Both have lots of white voters without college degrees, a bloc that has backed Trump in nearly every state. (About 26 percent of Ohioans and 27 percent of Floridians have bachelor's degrees, compared to 29 percent of all Americans.)
Both states will have a large segment of moderate and "somewhat conservative" GOP voters, hurting Texas Sen. Ted Cruz, who is strong among very conservative Republicans but weaker in other blocs.
But the home state factor could be very important. In Texas' primary, which Cruz won easily, he carried the kind of rural counties that Trump did in most other states.
Cruz and Rubio are not really competing in Ohio, so Kasich is effectively in a one-on-one race with the real estate mogul in the Buckeye State.
If Ohio Republicans simply want to support the governor they have elected twice, the demographics of the counties may not matter as much as in other states.
But if Ohio Republicans generally follow the pattern of other states, with lower-income and rural voters backing Trump, Kasich still has a path to victory.
Mitt Romney won this state in the 2012 primary by winning the areas in and around Cleveland, Cincinnati, and Columbus by large margins, even as he lost most of the counties in the state to ex-Pennsylvania U.S. Sen. Rick Santorum.
Romney carried 49 percent of the vote in Cuyahoga County, which includes Cleveland.
The other key is for Kasich to avoid being routed by Trump in the non-urban areas of the state. In Michigan's primary, Kasich received less than 20 percent of the vote in many rural counties, far behind the real estate mogul.
Will Florida voters save Rubio? This home-state candidate has deep problems. Cruz has opted to campaign in Florida, meaning the very conservative bloc of the electorate (33 percent in 2012) could back him. After Rubio's dismal performances in recent primaries, college-educated, upper-income Republicans in Florida who might have supported Rubio could opt for Cruz, Kasich or even Trump, viewing the Florida senator as a candidate with no realistic path to the nomination.
Rubio is likely to win in Miami-Dade County, which is the state's largest. That is where he grew up, started his political career and still lives. It also has a large population of Rubio's fellow Cuban-Americans.
But the other large urban areas of Florida, Orlando, Jacksonville and Tampa, are no guarantees for Rubio. Those cities do not have particularly large numbers of Republicans with college degrees who might be resistant to Trump.
Republicans in Columbus, Ohio have an obvious connection to Kasich: Columbus is the state capital, and Kasich works there and effectively employs those who work for the state's government.
In contrast, Rubio does not have deep ties to voters in Tampa or Orlando.
If Trump does not win Florida, it will be a major upset.
Trump Could Sweep Delegates in Illinois and Missouri Too
Illinois (69) and Missouri (52) reward a bloc of delegates to the statewide primary winner, but most delegates are allocated by congressional district. That said, if Trump runs strong statewide, he could win all of the delegates in both states, as he did in South Carolina.
Illinois' electorate will likely have one of the biggest groups of self-described moderate voters (36 percent in 2012) of any of the GOP states that have voted up to now.
Kasich is running as the champion of the GOP's moderates and its governing wing. If he is blown out by Trump in Illinois, it raises questions about his ability to take on the real estate mogul in moderate states like California and New Jersey in upcoming primaries.
Missouri will have substantial blocs of very conservative and evangelical voters, making it more friendly to Cruz. But if patterns from previous primaries hold, Cruz will lose there because of his weaknesses among moderate and somewhat conservative voters, who prefer Trump.
North Carolina awards its delegates (72) proportionally, so it is unlikely to have a huge impact on the nomination fight.
But Trump is the favorite there too. It's an illustration of Trump's broad appeal that he can be the favorite in states as different as Illinois, Missouri and North Carolina.
Sanders Needs Blowouts. It Will Be Hard to Get Any on Tuesday
Clinton's lead among delegates (766 to 551) is significant. Sanders last week won a key primary in Michigan, but lost ground among delegates because Clinton resoundingly defeated him in Mississippi that same day.
The large black electorate in North Carolina (34 percent of the vote in 2008) and the combined black-Latino vote in Florida (31 percent) mean Clinton is the favorite in those two states. Even if Sanders somehow won those states, his margin would likely be slight and do little to close the delegate gap. And white Democrats in the South have also tended to view Clinton more favorably than those in other regions, another advantage for her in these two states.
Illinois, Ohio and Missouri are big opportunities for Sanders. The black electorate will be likely between 15 and 25 percent of the vote in all three states. It would greatly help Sanders if he won at least 35 percent of black voters, as he did in Michigan, instead of 15 percent, as he did in several Southern states.
The areas in and around Chicago, Columbus, Cincinnati, and Cleveland will include some of the upper-income Democrats of all races who tend to back Clinton.
But Missouri in particular is a place where Sanders could win more than 60 percent of the vote.
Clinton could of course win more than 60 percent in North Carolina or Florida, therefore negating any resounding Sanders victories.