Washington(CNN) The magic number in the GOP primary is 848.
That's how many delegates Donald Trump needs to clinch the GOP presidential nomination.
It is also the number of delegates the other three candidates combined need to prevent him from clinching and to force a contested Republican convention.
It is quickly becoming clear that winning the nomination outright is largely out of the reach of any candidate not named Trump. While all are still mathematically viable, the climb is steep.
The best chance for any of them seems to be keeping Trump from clinching, creating a contested convention that would open up hope for other candidates.
But first, Florida Sen. Marco Rubio and Ohio Gov. John Kasich must win their home states to keep the cache of delegates out of Trump's hands. There are 99 winner-take-all delegates up for grabs in Florida and 66 in Ohio on March 15.
Based on CNN's delegate estimate after last weekend's primaries and caucuses, Trump needs to win about 54% of all the remaining delegates to clinch the nomination. Texas Sen. Ted Cruz, who trails by fewer than 100 delegates, needs to win 60%. Rubio, more than 150 delegates behind Cruz, needs to win about 69%, and Kasich would need to win 77%.
For reference, Trump has won about 44% of the delegates thus far.
"Is (keeping Trump from clinching) mathematically possible? Yes, it absolutely is," said Josh Putnam, a University of Georgia lecturer and election expert. "Is it realistically possible? I think a lot of that's going to be determined by whether or not Trump wins Florida and Ohio. If he wins both of those, gosh, it's hard to see how Rubio and Kasich carry on."
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Kasich and Rubio both understand the reality, pledging they will carry their states on March 15 and sinking heavy resources into doing so."Listen, when I win Ohio, and we're doing better in other northern states as well ... this thing is gonna go to the convention," Kasich said on CNN's "New Day" Friday. He is running a close second behind Trump in Ohio, 31% to 26%, just outside the margin of error, according to a February Quinnipiac University poll.
In Florida, the latest polling shows Rubio much farther behind. The most recent Quinnipiac University poll of Florida Republicans had Trump leading the local senator 44% to 28%.
Rubio has brushed off polling, promising unequivocally he will win Florida.
"We're going to win Florida," Rubio said Friday on ABC. "I know that poll is not accurate. I know for a fact."
He has also been happy to entertain talk of a contested convention, saying his entire campaign that his focus wasn't on winning states, but rather on picking up delegates.
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"What is possible is that no one has 1,237 delegates," Rubio said on "Fox and Friends" on Friday. "If you look at it now, even Donald Trump is going to have a tough time at this point getting the 1,237 delegates. ... I do not support a backroom deal at the convention, either. ... But I think this will all eventually work out and there will be a consolidation behind someone not named Donald Trump, and I'm telling you, that is going to be me."
Kasich meanwhile is focused on a Rust Belt strategy, where he is looking to drive up his numbers beginning with his home state of Ohio.
Ohio is critical for Kasich, as it is one of the first winner-take-all contests of the cycle. But the Kasich team has focused most of its firepower on Michigan, which votes Tuesday, for now -- spending roughly $850,000 on-air there, between the campaign and its affiliated super PAC, according to CMAG/Kantar Media. That's almost double the $440,000 Trump has spent on-air.
In Florida, as of Friday, Rubio's Conservative Solutions PAC had spent almost $5.3 million on-air in his home state. American Future Fund, which has featured the testimonials of alleged victims of Trump University, is spending close to $2 million there. And the Club for Growth, one of the first conservative groups to blast Trump, is spending a little more than $1 million there.
Trump has bought up close to $1 million on media in Florida, while Our Principles PAC -- the much-touted group packed with Jeb Bush staffers and establishment Republicans -- has bought up a little more than $440,000 on-air in Florida so far.
For his part, Cruz is still pitching himself as a nomination winner outright, highlighting that he is the closest in the delegate count to Trump.
"In my view, a brokered convention ain't going to happen, and if the Washington deal-makers tried to steal the nomination from the people, I think it would be a disaster," Cruz told reporters Friday. "We've got to win this nomination. One-thousand, two-hundred and thirty-seven delegates is what it takes to be the Republican nominee."
Putnam cautions that the math can be misleading, however. He said at this point in 2012, Mitt Romney seemed to be in a similar position to Trump, with an uphill climb to the magic number of delegates. But, he added, candidates drop out and the game rapidly changes.
Of course, Romney and Trump are different stories.
"This isn't a conventional cycle, but what we tend to see is a war of attrition," Putnam said. "(Traditionally) it's not about getting to a magic number, it's the race ending as a function of the last viable candidate dropping out. But that's where 2016 is different, there's a significant motivation from some within the Republican Party to prevent Trump from getting the nomination."