Why margin of victory will matter for Trump on Tuesday

The first test of the renewed "Stop Trump" movement in the Republican Party begins Tuesday with primaries in Michigan and Mississippi. The prospects don't look very good for those who would deny the reality TV star the nomination.

Donald Trump holds an average 12 point lead in Michigan, according to the latest polls and holds even larger leads in Mississippi. One poll conducted by ARG showed Ohio Gov. John Kasich with a narrow 2-point lead in Michigan. But that survey had a small sample size and looks like an outlier.

If Trump pulls off both wins, it would restore him to clear front-runner status and take some of the edge off the "surge" for Ted Cruz following the Texas U.S. senator's victories over the weekend in Kansas and Maine.

Republican presidential candidate Donald Trump addresses the crowd at a campaign rally March 7, 2016 in Concord, N.C.
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Republican presidential candidate Donald Trump addresses the crowd at a campaign rally March 7, 2016 in Concord, N.C.

A surprise in Michigan is not impossible. Kasich is clearly trending higher following his strong debate performance last week. But the anti-Trump vote remains splintered between Kasich, Cruz and Florida U.S. Sen. Marco Rubio. And much of the advertising by outside groups against Trump is focused on March 15 states, mainly Florida. Cruz's "Southern strategy" does not appear to have caught on in Mississippi where Trump dominates in nearly every poll.

So the narrative coming out of Tuesday night is likely to be Trump regaining traction. But that doesn't mean the effort to stop Trump from getting the 1,237 delegates he needs to lock down the nomination before the convention in July is dead yet. Because Trump does appear to be hitting something of a national wall as the impact of negative ads and stories about his cracking down on protesters, urging supporters to take a pledge reminiscent of the Nazi salute and engaging in unsavory business practices appear to be denting his national lead.

Trump's lead is now just 9 points over Cruz in the latest ABC News/Washington Post poll. Instead of consolidating support as other candidates drop out, Trump is falling while Cruz, Rubio and Kasich are all gaining ground. Perhaps even more significant, Trump trails both Cruz and Rubio in hypothetical one-on-one matchups.

While this Tuesday's results are important, the big game takes place next Tuesday with contests in Florida, Ohio, North Carolina, Missouri and Illinois. The outside groups targeting Trump are putting much of their money into ads in Florida. Polls currently show Trump leading in the state but the hope is that those who have not already voted early will change their minds by the time they hit the voting booths. Trump fared badly among late deciders in both Louisiana and Kansas over the weekend.

But again there is the problem of a splintered non-Trump field. Cruz is making a late play in Florida not to win the state but to deny a victory to Rubio in hopes that a home-state loss will drive the Florida senator out of the race. It's a very risky strategy because torching Rubio could help Trump maintain his lead and win the state and all of its 99 delegates.

And it is not at all clear that even if Cruz eliminates Rubio that he will be able to stop Trump as the race heads east to states like New York and New Jersey and later west to California. The Cruz strategy seems to assume that the establishment GOP will not let the Texas senator take the nomination at a brokered convention and that his only play is to win the delegates he needs before the convention. That may be an impossible dream for Cruz and attempting to attain it may only ensure that Trump gets the nomination.

Whatever the case, Florida is clearly the last stand for Rubio even if his campaign says they have no plans to drop out no matter the result. They have to say that. All campaigns deny plans to drop out until there is nowhere left to turn and the candidate has no choice but to exit the race.

Margins will matter on Tuesday night. If Trump wins Michigan but Kasich comes close, it will suggest the trend away from Trump is real. That will give donors unsure about writing checks to the various anti-Trump PACs incentive to put their money to work. But if Trump dominates both Michigan and Mississippi and appears headed to a big win in Florida, there may be no stopping him.

—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.