Photo Credit: Image by Shutterstock, Copyright (c) Joseph Sohm

Tuesday’s presidential primaries in five states is going to be the most important day so far for 2016 contests on both sides of the aisle—for nail-biting reasons.

On the Republican side, the likelihood that Donald Trump will stay on track to be the GOP’s 2016 nominee is in play. If he were to win everywhere, he’d have twice the number of delegates as Ted Cruz. But if he doesn’t sweep the most delegate rich states, Florida and Ohio (which Ohio Gov. John Kasich could win as a home-state favorite), the contest will go on in unpredictable ways.

An ongoing three-way contest of Trump, Kasich and Ted Cruz could end up helping Trump by keeping him in the lead as the other two split the remaining spoils. That’s what Princeton University Election Consortium co-founder Ted Wang persuasively argues in an analysis, “Losing Ohio Improves Trump’s Chance to Win The Nomination,” saying the numbers get harder for Trump in a two-way race.

On the other hand, political independents in states that have yet to vote could also become more important. While we have seen white working class voters go for Trump in blue states, the large percentage of voters who identify as independents could scramble the assumptions of many pundits that most voters follow party lines.

What is unfolding on the Democratic side is equally riveting. It’s not just because Bernie Sanders’ challenge to Hillary Clinton hangs in a balance as Florida, Ohio, North Carolina, Illinois and Missouri voters go to the polls—as midwestern big-state wins by Sanders will keep his presidential ambitions alive and in play. It’s also because losses by Clinton in more General Election swing states will underscore that her candidacy is vulnerable in must-win states for a 2016 presidential victory.

Anyone who is getting e-mail blasts from Sanders’ and Clinton’s campaigns is deluged with messages saying the stakes are getting higher and higher each day and with every upcoming state. Not counting super-delegates, who are elected officeholders and party oficials, Clinton has 772 delegates compared to 551 for Sanders.

There is another way to look at the fight for the Democratic nomination and that is through the lens of which states are likely to be the final battlegrounds next fall, the so-called swing states. Depending on which analyst you want to believe, there are seven to 10 states that will be in play—as opposed to being sure bets for Democrats or Republicans.

The swing states are: Iowa—where Sanders and Clinton tied; New Hampshire—where Sanders won big; Nevada—where Clinton squeaked ahead to win the first stage of the caucus process; Colorado—where Sanders won; and Virginia—where Clinton won. If we stop right there, so far neither Clinton nor Sanders has a swing-state edge. The bigger must-win states continue with Florida—where she is seen as leading as Tuesday’s vote approaches; and Ohio—where Sanders is seen as leading and also votes Tuesday. Also voting is North Carolina, which went for Republican Mitt Romney in 2012 and Democrat Barack Obama in 2008.

In other words, after today’s vote, Democrats will have a new benchmark to assess which candidate is the strongest when it comes to tipping the balance next fall, as most states yield predictable red or blue outcomes. This is why Sanders upset win in Michigan was such a stunner—not just because he was trailing in polls, or because Michigan is a reliably blue state for presidential candidates, but because it showed that Clinton did not own the electability argument; that only she is a national candidate who can win.

As of Tuesday, about half the states will have voted for a Democratic nominee. Most of the states that have voted so far have favored Clinton, such as her sweep across the South. But looking at the fall, almost all of those states aren’t going to backing a Democratic presidential candidate. They didn’t in 2008 and 2012. In other words, it doesn’t make a difference that Clinton has swept the South recently, with the possible exception of Florida, Virginia and perhaps North Carolina.

Similarly, as the primary season calendar moves to the West Coast and to rest of the Northeast, whether Clinton or Sanders prevails in California and New York is less important than who wins in two other states that now sit on a purple fence: Wisconsin, which has supported Democratic candidates in 2008 and 2012, and votes April 5; and Pennsylvania, which has also backed blue presidential candidates and votes April 26.

If Sanders wins Ohio and Illinois, while Clinton wins Florida and North Carolina—as some polls are predicting—it would not just show that he’s well-positioned for Pennsylvania and Wisconsin. (One Monday poll had Sanders ahead in Missouri). It also would show that Democrats are facing a big quandary over their nominee. Not only is Sanders running an arguably better campaign than Clinton—by building a come-from-behind record and growing momentum, splitting the swing states takes the race to a more contentious orbit.

It is easy to think that it’s just the Republicans facing a meltdown when it comes to 2016’s presidential election. But the Democrats are not in the clear. For differing reasons, neither Sanders nor Clinton are ideal candidates. He lacks her record of experience inside the executive branch, while she lacks his passion, moral compass and youthful supporters. He has a huge grassroots following while she has the party’s top insiders and establishment. These are big differences.

If Sanders and Clinton keep splitting 2016 swing states—and in the end somehow do not bring their campaigns together—it means the Democrats will enter the General Election with their own vulnerabilities, no matter whom the Republican nominee is. That’s why today’s vote in five states—Florida, Ohio, Illinois, North Carolina and Missouri—is more important than Super Tuesday two weeks ago. And there’s a key metric to watch beside the delegate tally: which candidate is winning more of next fall’s swing states.