Facing the possibility of a third-place finish in Monday's Iowa caucuses, Rubio will soon head to New Hampshire and battle in a state where polls show a heavily contested fight brewing among establishment-backed Republicans vying to become the alternative to Cruz and Donald Trump.
Of the establishment favorites, Bush is now viewed internally as Rubio's biggest threat after the New Hampshire primary, according to advisers and confidantes to both men.
Despite having run a rocky campaign while struggling in the polls, Bush is the only establishment candidate who has built a national organization that rivals Rubio's. It's Bush's high-spending super PAC, Right to Rise, which has pummeled Rubio across the airwaves with tens of millions of dollars in attacks -- and officials say it will continue to do so long as the former Florida governor stays in the race.
And if Bush continues his campaign beyond the Super Tuesday primary on March 1, as his confidantes promise he will, he could make it harder for Rubio to win what will be one of the biggest prizes in mid-March: Their home state of Florida, which awards all of its delegates to the winner rather than handing them out proportionally. The jockeying between Rubio and Bush -- one-time close political allies in the Sunshine State -- underscores what will soon become the dominant narrative after the Iowa caucuses: Who will become the choice of the party establishment?
"It will probably thin out to Jeb or Marco," said Rep. Adam Kinzinger, R-Ill., a top Bush surrogate. "You'll start to see some clarity in a couple weeks."
Rubio vs. the governors
The thinning-out process begins Monday night here in Iowa, and Rubio's team wants major separation between him and the governors in order to show that the senator is the only viable candidate who can take down Trump.
According to a CNN average of the last five public polls in Iowa, Rubio is solidly in third place here, coming in at 15% -- well behind Cruz and Trump. But from the perspective of Rubio officials, a third-place is finish is fine with them -- as long as they are solidly ahead of the other establishment favorites, namely Bush, New Jersey Gov. Chris Christie and Ohio Gov. John Kasich, who all will be competing for the No. 2 spot in New Hampshire.
So far, Bush, Christie and Kasich have failed to gain much traction in Iowa, all registering in the low single digits in the polls -- something that could hurt their momentum going forward. But as they've struggled, all three have increasingly focused on New Hampshire, hoping the state's more moderate voters help propel them to finish right behind Trump, who is dominating so far in the state.
A new CNN-WMUR poll released Sunday shows a muddled race for second and third place in New Hampshire. While Trump wins 30% of the vote, the next closest competitor is Cruz at 12%, followed by Rubio with 11% of the vote, Kasich at 9%, Christie at 8% and Bush at 6%. Just 39% of GOP voters say they've "definitely" decided on their candidate.
From the Rubio perspective, they believe the senator can afford to finish behind Kasich in New Hampshire, who lacks much of an infrastructure beyond the Granite State and doesn't have deep support from the conservative base. While Christie does have greater support among conservatives, according to surveys, he too lacks much of an operation in states that vote after New Hampshire.
Bush, on the other hand, has devoted ample time and resources traveling across the country, including last week where he went to the remote town of Elko, Nevada -- even though the state doesn't vote until Feb. 23. Bush and Rubio have traveled to states that vote in March in a much-faster clip than their rivals. Since mid-June, Bush has made 49 trips to March-voting states, compared to 31 to Rubio. Bush's campaign is investing heavily in Tennessee, a state that votes March 1, and has built coalitions in states as diverse as Colorado and Alabama.
Even if they fall short in New Hampshire, they are increasingly banking on a big showing in South Carolina to turn around a campaign that has underperformed so far.
"South Carolina is a really natural fit for Jeb," said Kinzinger.
Even if Bush struggles to break through in New Hampshire, his well-funded super PAC, Right to Rise, can keep him afloat -- and make Rubio's life difficult.
The Bush super PAC has already spent $20 to $22 million attacking Rubio, according to the senator's campaign. Paul Lindsay, a spokesman for the super PAC, wouldn't confirm that number. But he signaled that it was just a start.
"It's less than the figure they quote, but we'll cut Marco some slack since we know he has a penchant for not being able to add up expenses very well," Lindsay said. "We are very prepared for March and believe Jeb will be in a strong position at the end of February."
The Rubio organization and the Cruz factor
The Rubio campaign has built an organization that the senator's team believes can sustain well beyond the first four states, even if he fails to win any of them. They've named campaign chairmen in all 50 states and have secured more state legislative endorsements than Bush since late December, totaling 228. The campaign has opened up three offices in South Carolina and two in Nevada, and now has 118 staff -- more than half are working on targeting voters on-the-ground.
Rubio is spending money on TV in Nevada, unlike other campaigns, and is increasingly banking on a strong showing in the Silver State. Since December, Rubio also has traveled for events in Alabama, Florida, Michigan, Missouri, North Carolina, Texas and Minnesota.
And both Rubio and his campaign will likely have the resources to compete into March. His super PAC, Conservative Solutions PAC, reported $14 million in its bank account through the end of last year.
Jeff Sadosky, a spokesman for the super PAC, said roughly 55 percent of all negative ads in Iowa were targeting Rubio -- and that the organization had the resources to respond in kind.
"That air cover has been incredibly important as Marco has found himself attacked by Jeb Bush, Ted Cruz and Hillary Clinton," Sadosky said of his organization's air war.
The Rubio strategy comes down to this: They hope that they will continue to advance by staying in the top tier of candidates, while under-performing candidates -- namely the governors -- face growing pressure to drop out, allowing him to consolidate their support. They hope that will allow the party to unite behind his candidacy to take on the conservative favorite, potentially Trump.
On Sunday, Rubio refused to say which state he could win when asked by CNN's Jake Tapper on "State of the Union," saying he's "not the campaign strategist." And while he said that he wouldn't call on anyone to drop out of the race, he added that unity behind an establishment favorite would happen at the "appropriate time."
"Here's how we win: We win by having more delegates than anyone else, and more than half the delegates," Rubio said on NBC's "Meet the Press," signaling a long, grueling campaign. "And I'm very comfortable and confident that we're going to achieve that."
On Monday, the first task is seeing Cruz come in second to Trump in Iowa, something Rubio advisers believe will mean the beginning of the end for the Texas Republican's campaign. Yet if Cruz pulls off a victory Monday night, he will have upended the race, prompting Rubio and other campaigns to recalibrate.
No matter what, Cruz said, he'll be in the game for the long run.
"We're running a national campaign," Cruz told Tapper Sunday.