What does Rubio need to win the Republican nomination?

Donald Trump is bulldozing through the GOP primaries. He's racked up decisive victories in three out of four states, 79 of the 118 delegates so far apportioned among those left in the GOP race, and the kind of momentum that would inspire envy in any campaigner. All of which has political observers asking one question: How can the GOP can stop him?

The answer, according some, is the GOP's best chance at an "anti-Trump" last candidate standing: Marco Rubio.

"Rubio is clearly the most electable candidate against the unlikable Hillary Clinton because he’s an optimistic, likable candidate of the mainstream conservative movement without Ted Cruz’s oleaginous affect or Trump’s lunatic bluster," Rick Wilson, a national Republican message and media strategist, told Politico.

His strengths are obvious – the junior senator from Florida is a young, eloquent, Latino with an inspiring rags-to-riches story and an optimistic message. But his window of opportunity is closing.

"Rubio needs to do something to dramatically alter the dynamic of the race, and he probably needs to do it by [Super] Tuesday," says David Ryden, a professor of political science at Hope College in Michigan.

Things have already started to fall into place for Senator Rubio: He's spun a third-place finish in Iowa and second-place finishes in Nevada and South Carolina as victories; rival Jeb Bush dropped out, giving Rubio's campaign an important boost; and another rival, Mr. Cruz, is struggling in the south, purportedly Cruz country, opening opportunities for the Floridian.

But a lot more needs to happen for Rubio to have a shot at beating Mr. Trump.

More rivals need to drop out

"The biggest thing that would help Rubio is for Ted Cruz and John Kasich to drop out," says Whitney Ross Manzo, assistant professor of political science at Meredith College in Raleigh, North Carolina. "John Kasich is capturing a lot of moderate voters, and I would bet they would swing to Rubio if he’s gone. That leaves Ted Cruz...if he left that could unify the party behind the 'anti-Trump.' "

It's not surprising that party leaders are pressuring poorer-performing rivals such as Gov. Kasich and Ben Carson to drop out. According to conventional wisdom, they're an anchor around the necks of stronger rivals, and the sooner they drop out, the sooner the party establishment can coalesce around a challenger to Trump.

"Donald Trump can absolutely be stopped, by rallying the party’s anti-Trump voters around a single figure," Bill Scher, senior writer at the Campaign for America’s Future, told Politico. "Give Marco Rubio the Jeb Bush and John Kasich voters in South Carolina, and he would have beaten Trump by five points."

But, adds Professor Ryden, Rubio may actually benefit from Senator Cruz staying in the race. "I think Cruz needs to stay in the race for Rubio's sake. I believe his departure would help Trump more than Rubio, and he's an able attacker of Trump," he says.

Rubio needs to emphasize electability

"Rubio also needs to hit hard on the electability platform. Almost every hypothetical poll has Trump losing big to either Democratic nominee, so Rubio needs to say over and over that he’s the only one who could beat the Democrat in the general election," says Professor Manzo. "This won’t change the mind of Trump supporters, but it is definitely on the mind of every other Republican."

In fact, considering that polls show Rubio beating Hillary Clinton in a hypothetical match-up, and Trump losing, electability is one of Rubio's strongest selling points.

Rubio needs to attack Trump

"He has no choice but to take the gloves off and go at Trump directly," says Ryden. "He also needs as much of that establishment money as is out there to get anti-Trump ads up in as many Super Tuesday states as possible. He may not need to win states, but he needs to significantly close the margin between him and Trump to create some sense of momentum for the big March 15 contests."

On March 15, there are Republican primary votes in Florida, Ohio, Illinois, Missouri, and North Carolina.

It's a prescription conservative political analyst Bill Kristol echoed on Twitter.

Should these conditions be met, Rubio could presumably do well in some of the upcoming Southern contests on Super Tuesday and beat Ted Cruz in northeastern primaries where voters want someone who appears to be moderate and reasonable, to give him a shot at beating Trump.

Even then, major questions remain.

When will he win a state?

Rubio hasn't won a single primary state yet, and it's unlikely he'll win any of the upcoming contests in southern states where Trump and Cruz are expected to do well. Worse still, it's possible the junior senator won't even win his home state of Florida where January polls showed Trump up between 12 to 32 points.

Will his rivals drop out in time?

Mr. Kasich is siphoning some supporters of Mr. Bush and possibly drawing enough votes away from Rubio to cost him delegates in northeastern and midwestern states. Ditto Cruz, whose relative strength in the South may cost Rubio more southern state delegates. The longer they stay in the race, the more Rubio has to lose.

Can Rubio survive a Trump attack?

Because of course, if Rubio is the last Trump rival standing, it will eventually come to that.

"Trump will be far more unscrupulous once he sets his sights on Rubio. He will be willing to play to GOP base voters' worst ethnic and racial fears," writes Vox. "We don't yet know whether Rubio can beat Trump in a knife fight. But, to win the nomination, that's what he's going to have to pull off."

The big question, then: Can Rubio can survive what others – including Carson, Cruz, and Bush – could not?