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Is Marco Rubio the New Favorite to Win the GOP Nomination?

A series of strong performances on the campaign trail and missteps by other candidates have helped improve Marco Rubio's presidential prospects, and some top party operatives say the Florida senator is for now the favorite to win the GOP presidential nomination.

Sen. Marco Rubio.
Mary Stevens | CNBC
Sen. Marco Rubio.

The struggles and eventual withdrawal of Wisconsin Gov. Scott Walker eliminated one of the Republicans who, like Rubio, had the potential to appeal to both the establishment and conservative wings of the party. Former Florida Gov. Jeb Bush has not overcome reluctance from the GOP to elect a third president from the same family and instead has raised concerns that didn't exist about him at the beginning of the year.

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Despite his reputation for being smarter and more polished than George W. Bush, Jeb Bush has made a number of gaffes on the campaign trail, most recently when said he would not offer African-Americans "free stuff," in reply to a question about his potential appeal to black voters. And even those hostile to Donald Trump say that the real estate mogul has accurately cast Bush's campaign style as "low-energy."

Senior Republicans say they still think it is unlikely the party will turn to Trump or Ben Carson, who have led in many polls but have little political experience and have made a number of controversial statements that could hobble them in a general election.

Rubio, meanwhile, has impressed Republicans with his performances in the two debates and his relatively error-free campaign. In particular, in the debate at the Reagan Library in California, Rubio, speaking about U.S. policy in Syria, said, "Here's what you're going to see in the next few weeks: the Russians will begin to fly combat missions in that region, not just targeting ISIS, but in order to prop up Assad."

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Rubio has portrayed himself as a foreign policy expert, and his prescience in predicting the behavior of Vladimir Putin has drawn praise in conservative media outlets like Breitbart.

"The most likely person is Rubio to be able to coalesce both parts of the party and have a unified Republican base," said Gary Marx, a longtime Republican strategist who was aiding Walker's campaign. "I think that's the very best chance of all the candidates."

Republicans say Rubio must now clear several hurdles. As he appears to be one of the most likely Republicans to win, media scrutiny of Rubio's years as member and eventually speaker of the Florida House of Representatives and as a U.S. senator will increase. One longtime party donor who spoke to NBC News said he wanted to see how Rubio handled that scrutiny before committing to back him.

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Marx said it is not yet clear if Rubio has the ground operations to win early states, particularly the caucuses in Iowa and Nevada.

And while Republican voters did not warm to Walker, other candidates are viewed more favorably and could eclipse Rubio. Republican officials say there is considerable enthusiasm among both grassroots and establishment Republicans for Carly Fiorina , who is an outsider to elected office but has demonstrated knowledge of policy issues that Carson and Trump lack and has some insider credentials from having served as an adviser to John McCain's 2008 campaign. And party officials say that New Jersey Gov. Chris Christie or Ohio Gov. John Kasich could win New Hampshire and use that as a springboard to the nomination and that Texas Sen. Ted Cruz could do the same in Iowa.

Carson and Trump could win if voters decide they are fed up with traditional politicians.

In the wake of Walker's departure from the race, some of his supporters have gone to Rubio, but many have embraced other candidates, Bush and Cruz particularly.

Read MoreGov. Scott Walker quitting GOP race

Another potential problem for Rubio is that his rivals already see him as a clear, serious threat and are highlighting his two vulnerabilities: his age and immigration stances. Bush is now likening the 44-year-old Rubio to President Obama, arguing neither man had sufficient executive experience to be president.

Trump is casting the senator as "Marco 'amnesty' Rubio," because of Rubio's 2013 membership in the "Gang of Eight" in the Senate that favored an immigration bill that included a path to citizenship for undocumented immigrants.

"I watched lightweight Senator Marco Rubio, who is all talk and no action, defend his WEAK position on illegal immigration. Pathetic!," Trump said in a Twitter message last week.

But Republicans say Rubio may be able to overcome his immigration challenges. The senator has now withdrawn support from the 2013 immigration bill, arguing any such legislation should be divided into small parts, not one large bill. The additional immigration enforcement provisions, Rubio now says, must come before any kind of path to legalization for the undocumented.

And Bush, the best-funded GOP candidate, may be ill-positioned to run a negative ad campaign to take down Rubio, because the two have essentially the same position on a number of issues, including immigration.

"They do share a lot in common, they are friends, they are ideologically aligned on most issues," said Hector Barreto, who was the head of the Small Business Administration under George W. Bush and then was a co-chair for Mitt Romney's Hispanic Leadership Team in 2012.

In part, the rise of Rubio simply reflects declining Republican optimism about Bush, who was considered the party front-runner when he entered the race. Bush has struggled not only in polls, but also in recruiting prominent Republican elected officials to back him. Unlike the 2000 campaign of George W. Bush, Jeb Bush's advantage in fundraising so has so far not indicated any broader support among Republicans.

"His donors are going to have to accept that he is not going to make it," said one longtime GOP operative of Bush, describing Rubio as the best Republican candidate.

Rubio and his team are wary of the emerging buzz about his prospects, aware that it will increase scrutiny of the candidate.

"There would be nothing worse in mind than being in first place right now. It's terrible," said Terry Sullivan, Rubio's campaign manager, at a recent briefing with reporters.

He added, when a candidate leads in polls, "The New York Times writes stories about how big the windows are in your house and how well-manicured your yard is."

Sullivan said the Rubio campaign's strategy is two-fold. Use the debates and other forums to position Rubio as a serious-minded, policy-oriented candidate.

More on the Republican presidential debate coverage

"We are building this for the long haul," he said.

Rubio "is not going to make headlines every day," Sullivan added. "He is not going to be the guy at any debate that comes up with the best one-liner of the debate, just not going to be him. He's going to be the guy over the course of the debates, who say you say, 'you know what, I'm kind of comfortable' .... Steady wins the race."

Secondly, Sullivan listed the names of past Republicans nominees George W. Bush, Bob Dole, John McCain and Mitt Romney. Those four, Sullivan argued, positioned themselves during the primaries as candidates who could appeal in all states and to all wings of the GOP. Rubio will follow a similar model, trying to reach social, economic and foreign policy conservatives and both the establishment and the Tea Party.

"We're not a niche candidate, where we only have one lane and we're going to really double-down on that lane," said Sullivan. "We also don't scare anybody. Yes, you have to become the first choice of enough people. But the pathway to do that is to not be scary to any part of the party. There are die-hard Ted Cruz supporters who think "yeah, I like Marco Rubio" and there are die-hard Jeb Bush supporters who say "yeah, I like Marco Rubio."

"That's important," he concluded.

That approach seems wise. At the same time, Walker was making the same argument, that he could appeal to all wings of the Republican Party. He left the race last month, having failed to energize many Republicans of any belief.

Rubio will have to avoid a similar fate.