"I would probably go with Chris Christie," said Duffy, a right-of-center Wisconsin Republican.
The comments are a sign of how Rubio and Christie are battling for the same profile of voters ahead of the New Hampshire primary next month: Moderates and right-leaning but establishment-minded Republicans who are looking for an alternative to Donald Trump and Ted Cruz.
Rubio and Christie each appear to now view the other as perhaps their clearest foe ahead of next month's primary in the Granite State. Confidantes of both men believe the victor of the clash will be able to make the case to the anti-Cruz and anti-Trump wing of their party to quickly consolidate behind the winner, adding considerable momentum headed into the South Carolina primary. The loser could find himself struggling for cash and momentum -- and facing pressure to drop out of the race and get behind a unity candidate.
And if both men lose to the other candidates vying for establishment support -- namely former Florida Gov. Jeb Bush or Ohio Gov. John Kasich -- they both will be severely damaged heading into a critical stretch of primaries.
On Thursday, the weeks-long battle between the two men, which intensified this week with a hard-hitting ad from Rubio's super PAC dubbing Christie as "Obama's favorite Republican" and Christie later saying that Hillary Clinton would "cut his heart out," continued to grow nasty.
"Chris Christie has done a number of things that are very similar to the Obama agenda," Rubio said on Fox Business Network on Thursday. "And I just don't think this country can afford to elect a president that will not stand up and undo the damage Barack Obama has done to the country."
Rubio also added on "Fox and Friends:" "Our president has to be someone who is a consistent, long-time defender of our Second Amendment -- not someone with a questionable record on it," referring to Christie.
Samantha Smith, a Christie spokeswoman, fired back.
"This is the difference between a senator and an executive," Smith said. "While Sen. Rubio has no doubt given some nice speeches on these issues, Gov. Christie has vetoed Planned Parenthood funding, ended Common Core in New Jersey and protected the rights of gun owners."
The rise of Christie has been celebrated perhaps no more exuberantly than in Houston, where Cruz's campaign sees Rubio as more and more pinched. Cruz, who has not campaigned in New Hampshire as aggressively as other rivals, has essentially dared Rubio to win it, saying that as the moderate front-runner, he is supposed to do the best in the more moderate state.
And it's not just in New Hampshire: In Iowa, where Christie is making a beneath-the-radar play for a solid showing, Cruz's camp is quietly rooting for Christie, someone the Texas Republicans' allies believe would be easier to dismantle.
"If Christie can actually do something here in Iowa and place above Rubio, than he's the story going into New Hampshire," said a Cruz adviser, granted anonymity to discuss campaign strategy, crowing that Rubio's super PAC ad shows his new problem. "He strategically believes that Christie is a threat, so he has to deal with it."
Nervousness over a divided establishment
Sources close to both men say they want to use a second-place New Hampshire finish -- likely behind Trump -- to pitch themselves as a consensus candidate to the party's establishment wing. It gets much harder if one of them loses to the other. And already, GOP moderates are growing very eager to unite behind an establishment favorite and push back the rebellion from the conservative base.
"If you're a member of Congress in a swing district or a senator in a swing state, you have to be prepared to put some distance between yourself and the top of the ticket -- depending on who it is," said Rep. Charlie Dent of Pennsylvania, a leading House GOP moderate, referring to Cruz and Trump. "I think somewhere between Rubio, Kasich, Christie, Bush and [Carly] Fiorina would be ones would likely to be helpful to down-ballot candidates."
In CNN's most recent New Hampshire poll
, released in early December, Rubio was in second place in New Hampshire to Donald Trump, 32% to 14%. Right behind Rubio was Christie, at 9%.
Two other recent polls, from the Boston Herald and WBUR, put Cruz, Christie and Rubio in a statistical dead heat for second place at 10-12%.
Christie, however, has tried to put some distance between him and Rubio.
Speaking to reporters in Exeter, New Hampshire, last month, Christie whacked Rubio for skipping a December vote on a $1.8 trillion omnibus spending bill that the Florida Republican had vowed to battle.
"Sen. Rubio, listen if you going to say you are opposed to something, how about showing up to work and voting 'no,'" Christie said.
But Rubio and his allies have pushed back, noting how Christie has skipped work in New Jersey and has been berated by bad press in the Garden State.
"There hasn't been a day when I've seen Chris Christie in the state of New Jersey," Rep. Tom Rooney, R-Florida, a Rubio supporter, said Friday. "It's a little bit of the pot calling the kettle black."
Rubio's two-front war
For Rubio, his path starts in the Iowa caucuses on February 1, where he is pouring more money across the airwaves than almost any other candidate. Rubio's team does not believe he needs to defeat Cruz or Trump in the Hawkeye State.
But they believe he needs to come in a strong third place, one reason why they are slashing Cruz in an effort to peel away some of his supporters. A strong third-place finish would give Rubio momentum headed into New Hampshire, a chance to take out Christie, Bush and Kasich.
Rubio's campaign believes that the freshman senator is the one candidate who can appeal across the different segments of the GOP coalition.
In his stump speeches, he tries to portray himself as a tea party outsider, talking about how he defeated the establishment-backed candidate in his 2010 Senate race, Charlie Crist. He tries to appeal to the party's defense hawks by calling for an expansive U.S. role in the world powered by military might, something that could also appeal to New Hampshire Republicans. Rubio makes sure not to make polarizing remarks, telling a voter this week in Burlington, Iowa, "I wouldn't go that far," when the voter called Obama a "Marxist," a nod to a cautious party establishment.
And he speaks openly of his devout Roman Catholic faith, an effort to peel away conservative Iowa voters. "Our rights come from our creator -- they don't come from the government," Rubio said in Marshalltown, Iowa, this week.
The effort comes at a risk: Without a clear lane, he could be stuck waging a two-front war -- and lose both.
"Strategically, the first step in Rubio's journey needs to be to consolidate the available establishment type vote that is concerned about Cruz and terrified of Trump," said Bruce Haynes, a GOP strategist. "He has not claimed control of it yet the way that Trump and Cruz have claimed their lanes."