This has been a good week for Marco Rubio. After finishing a close third in the Iowa caucuses, there are early indications that the Florida senator is gaining ground among GOP primary voters nationally. One survey, from Public Policy Polling, found that although Donald Trump still leads with the support of 25 percent of Republican voters, Rubio is now tied with Ted Cruz at 21 percent. Better still, PPP finds that it is Rubio who would benefit most if other candidates drop out. He leads in hypothetical head-to-head matchups with both Trump and Cruz, and he’s also narrowly ahead in a three-way race. He’s often accused of running a tightly scripted, ultra-cautious campaign, but it looks as though Rubio’s strategy is being vindicated.

Of course, polls are just snapshots, and there is no guarantee that Rubio won’t stumble in the weeks and months to come. The New Hampshire primary is fraught with danger for Rubio, largely because Jeb Bush, Chris Christie, and John Kasich are so desperate to do him harm. Right to Rise, the super PAC backing Jeb Bush, has spent millions deriding him as an unprincipled political weather vane. Christie has been even more aggressive over the past few days, mocking Rubio as “the boy in the bubble” and portraying him as an extremist on abortion, previewing an attack that Democrats are certain to use if he emerges as the GOP nominee. Liberal commentators like Dana Milbank of the Washington Post have been no less disdainful, seeing Rubio’s refusal to mix it up with Trump as a sign of cowardice rather than a marker of discipline and a desire to remain focused on his own more optimistic message.

Are Rubio’s critics right? Is he a chameleon, someone who will abandon his convictions in pursuit of his political ambitions? That’s a bit harsh, but there’s some truth to the notion that Rubio has a knack for being all things to all people. It’s Rubio’s chameleonlike nature that makes him the Republican candidate who’s best positioned to win a general election and to meaningfully advance conservative objectives once in office. The irony is that it’s also what makes Rubio so vulnerable in the Republican primaries.

Rubio first emerged on the national scene in 2010 as the conservative challenger to Charlie Crist for a Florida Senate seat. Opposing a popular Republican governor was a bold move, and throughout his Senate campaign, Rubio took Crist to task for failing to adhere to conservative principles by, for example, embracing President Obama’s fiscal stimulus law. Backed by the burgeoning Tea Party movement, many assumed that Rubio was a man of the hard right. But while other Tea Party conservatives were quick to condemn the president as an un-American socialist, Rubio took a gentler approach, as Mark Leibovich of the New York Times observed at the time. “[F]or the most part,” Leibovich wrote, “Rubio’s rhetoric is not harsh or personal against the president—less so than I would have expected given his firebrand following.” And when Leibovich asked him about his growing celebrity in conservative circles, Rubio demurred, explaining that he was “not a fan of personality-based politics,” which he found “very third worldish.” Rubio told Leibovich that “[p]eople who pin their trust and faith in a person are bound to be disappointed.” He referred to himself as “just a messenger for a set of ideas.”

What’s so special about a Tea Party conservative defining himself as a messenger? To understand Rubio, one must appreciate that he sees himself not simply as an ideological vessel but as an evangelizer, a politician who wants to win converts to conservatism. In one of the more uplifting passages of the speech he delivered right after the Iowa caucuses, Rubio promised “to take our message to the people who are struggling paycheck to paycheck [and] to the students living under the burden of student loan.” These are the very same voters who rejected Mitt Romney and John McCain in large numbers. Now, Rubio wants to enlist them in the conservative cause.

This is by far the most compelling argument for the Rubio campaign—that unlike most of his rivals for the GOP nomination, he knows how to sell conservatism to nonconservatives. It doesn’t hurt that Rubio is a convert himself, a liberal firebrand as a youth who once marched in union picket lines with his father. Quite unusually for a Republican presidential candidate, Rubio also cut his teeth in local politics in a lower-middle-class, overwhelmingly Latino suburb of one of America’s biggest cities. Earlier in his career, Rubio contemplated running for mayor of Miami-Dade, a job that requires a deep understanding of nitty-gritty issues like how to pick up the trash and what to do about potholes. Urban politics are anti-ideological almost by nature, and the fact that Rubio had to win over voters in this context speaks well of his ability to square circles. While pushing his 100 Innovative Ideas initiative, a grab bag of proposals for improving Florida’s government that he dreamed up while serving in the state Legislature, Rubio offered the following thoughts:

I come from what people in the political mainstream would call “right of center.” I am not anti-government. I believe that government is an important institution in a successful society—I just do not believe that it is the most important institution. Government’s job is to create regulations that protect public safety and the environment. To provide help to parents, families, and the civil society in raising children. To provide the rule of law needed to have a robust and competitive economic climate. To provide a safety net to catch people who get lost in economic competition so they can stand up and try again. And to help those who are elderly, disabled, or sick and cannot provide for themselves.

There is nothing intrinsically unconservative about what Rubio writes in the passage above. Believe it or not, most conservative Republicans believe in a government that is capable of performing its core functions competently. Yet it’s clear that Rubio is not preaching to the converted. He is speaking, or at least trying to speak, to middle-income swing voters who are skeptical of reflexively anti-government conservatives. He is making it clear that his brand of conservatism was far more pragmatic and reasonable than many youngish, middle-class Latinos in South Florida had come to expect.

The more conciliatory language that evangelizers use to win converts can sow doubt among true believers, who start to question the purity of the missionaries in their midst. As a consequence, many on the right are convinced that Rubio is a RINO-in-waiting—a conservative of convenience who will sell out his ideological allies the moment it is convenient for him to do so.

Why would conservatives believe that Rubio is a RINO when he has one of the most conservative voting records in the U.S. Senate? Why is the onetime Tea Party favorite widely regarded as the leading moderate alternative to Trump (who is to Rubio’s left on most issues) and Cruz (who has an almost identical voting record)?

The most obvious cause is Rubio’s changing stance on immigration. After staking out a strong anti-amnesty position in his 2010 Senate race, Rubio joined forces with Democrats and moderate Republicans to craft comprehensive immigration legislation. Among other things, that legislation granted legal status to the vast majority of unauthorized immigrants while increasing immigration levels, both of which are highly unpopular among conservative activists.

The explanation for why Rubio made such a sharp shift is fairly straightforward. In January, Eliana Johnson of National Review contrasted the diverging paths of Rubio and Cruz, two senators elected with Tea Party support who’ve built their presidential campaigns on very different understandings of the American electorate. Essentially, Cruz came to the conclusion that the 2016 presidential race would resemble the dynamics of his own insurgent campaign in Texas in 2012. By presenting himself as an aggressive anti-Washington outsider, he could energize enough conservative voters, and in particular enough white working-class men, to win in November. Rubio, meanwhile, believed that the next Republican nominee would have no choice but to bring new voters into the GOP tent and that comprehensive immigration legislation that placated pro-immigration activists was an important part of getting there.

What Rubio failed to anticipate is that immigration—which had long been an issue where opinions did not neatly divide along partisan lines—would emerge as a test of party loyalty. As Ramesh Ponnuru has argued in Bloomberg View, “immigration is rapidly becoming a defining issue for American conservatism.” Those who take a hard-line view are comfortably in the conservative camp while those who do not increasingly find themselves on the outs. As a result of these shifting political winds, Rubio is now racing to the right on immigration, claiming that the rise of ISIS is such an epochal event in the history of human affairs that it makes his earlier commitment to the Gang of Eight comprehensive immigration legislation meaningless. Given that Islamic extremism was very much on the radar the entire time Rubio worked with Chuck Schumer and others on an immigration bill, this ISIS-changed-everything stance beggars belief.

Whether or not Rubio manages to convince GOP voters to forgive his immigration heresies, his troubles there reflect a larger problem. A number of observers, including Daily Beast columnist Eleanor Clift, have compared him to Bill Clinton, a politician singularly gifted at changing his positions to fit new political moods and moments. Over the course of his presidency, Clinton whipsawed from praising Germany’s social market economy to seriously contemplating Social Security privatization. He embraced calls for reducing legal immigration, and he later took Republicans to task for doing the same. Clinton famously raised taxes on the wage income of the richest Americans before less-famously signing off on deep cuts to their taxes on capital gains. And though there have always been ideological liberals who detested Clinton’s perpetually shifting principles, including many of the activists who fueled the candidacies of Howard Dean, Barack Obama, and now Bernie Sanders, Clinton has nevertheless long commanded the allegiance of rank-and-file Democrats.

The Clinton-Rubio comparison doesn’t quite work, both for reasons of scope and the resulting political outcome. The Florida senator, unlike our 42nd president, has flip-flopped on just one big issue: immigration reform. At this point, it’s possible that this single flip-flop could doom his candidacy.

How could Clinton get away with changing his views constantly while Rubio has been punished far more severely for much smaller-scale maneuvering? It’s more than just a difference in political talent or experience, though both factors may well play a role. While being Clinton-like is a great way to win over Democrats, it’s an entirely different story for Republicans. As political scientists Matt Grossmann and David A. Hopkins have found, Democrats are best understood as a coalition of interest groups that seek government action on their behalf. For Democrats, half a loaf is always better than none, and so the fact that Clinton moved the ball forward on core party priorities meant that Democrats were willing to forgive his occasional ideological deviations. In contrast, the GOP is more like the vehicle for an ideological movement, and its adherents are far more likely to take an all-or-nothing view. That’s an awkward fit for a politician like Marco Rubio.

Rubio’s candidacy also isn’t helped by the fact that we don’t really know how he would govern in practice, as he has never served in an executive role. Whether you love or hate Rubio, you have no choice but to project your own beliefs onto his blank canvas, because he has only been in national politics for a little more than five years.

It happens that I hold Rubio in high regard, for his intellect, his empathy, and his understanding of modern, urban, 21st-century America. Like me, Rubio is a second-generation American, a man whose love of his country is profoundly shaped by his immigrant parents. But I can absolutely see why many conservatives can’t quite trust the junior senator from Florida, and why he has a long road ahead if he wants to earn that trust.

Read more Slate coverage of the GOP primary.