Tuesday could be the last stand for the "stop Trump" movement in the Republican Party. If the real estate mogul manages to win Florida and Ohio and their combined 165 delegates, there is probably no stopping Donald Trump from wrapping up the nomination before the party's convention in Cleveland in July.
Ohio looks like the establishment's best hope. Polls show the race either tied in the Buckeye state or give Ohio Gov. John Kasich a slight edge. He has the backing of the state party establishment and is campaigning on Monday with Ohio GOP Sen. Rob Portman and 2012 GOP nominee Mitt Romney. If all of this is not enough to keep Ohio from Trump, then nothing can stop him.
Florida, which like Ohio is winner take all for the GOP, looks like a near lock for Trump, with polls showing him up an average of nearly 20 points over home state Sen. Marco Rubio. If Rubio pulls off the shock victory for the 99 delegates, it would be yet another blow for the polling industry. The Rubio campaign hopes to run up big numbers in South Florida to counter Trump's strength to the north but Sen. Ted Cruz of Texas has made a late play to try to deny Rubio the win and drive him from the race. He may succeed in doing just that. Cruz has even edged ahead of Rubio in some Florida polls. A third-place finish in the state would be a devastating embarrassment for Rubio.
If Kasich wins Ohio and Trump wins Florida, supporters of the effort to stop Trump still believe they can keep him from getting to 1,237 delegates before the convention and ultimately deny him the nomination. Cruz is relatively close to Trump in two other states voting on Tuesday, Illinois and Missouri, and could collect some delegates in each state as well as in North Carolina. Kasich could also pick up some delegates outside Ohio on Tuesday.
After Tuesday, the GOP race turns to the West and Northeast. Trump could do well in many of these states but he is by no means a lock. Cruz, for instance, led Trump in a California primary poll taken in January. The California primary isn't until June and has rarely mattered in recent GOP history. But if it's a race to block Trump from getting a majority of delegates, it could matter in 2016.
California has 172 delegates and is a winner-take-all state. Many but not all of the remaining GOP primary states are winner take all as well. Trump opponents note that some of the bigger remaining contests including New York, Wisconsin, Connecticut, Oregon and Washington state all award delegates on a proportional basis. Trump could win some or all of them but perhaps not convincingly. The theory of the case for Trump opponents funding ads against him is that if he splits Florida and Ohio he would need to run up delegates at a pace far greater than he is now in order to get to 1,237 by July.
So far, Trump has not been able to consolidate his support into a majority of the GOP primary electorate. More Republicans continue to vote for non-Trump candidates than for Trump. If that continues, Trump will go into the convention without the delegates he needs and could face losing the nomination to a consensus candidate after multiple ballots. And all the chaos at recent Trump events could serve to both harden the billionaire's core support while also limiting his ability to break out above 50 percent and lock down the nomination.
On the Democratic side, Hillary Clinton is once again set for big victories according to the polls. But she was also viewed as a near lock in Michigan last week and lost to Vermont Sen. Bernie Sanders. So anything can happen. Sanders has closed in on Clinton in Ohio and a win there would extend the race and likely force Clinton even further to the left on trade and other issues. Illinois and Missouri are also close. So outside of Florida, it's possible that Clinton could wind up having a rough night on Tuesday and a Democratic race most thought would be over early will extend well into the spring.
If Trump splits the two big states, establishment dreams of keeping him from the 1,237 delegates he needs will remain viable. If he somehow loses both, Trump could be in real trouble. Trump could take the other big prizes on Tuesday, but Kasich has a shot in Illinois and Cruz has his best chance in Missouri.
—Ben White is Politico's chief economic correspondent and a CNBC contributor. He also authors the daily tip sheet Politico Morning Money [politico.com/morningmoney]. Follow him on Twitter @morningmoneyben.