Mr. Walker is the best-positioned candidate to pull it off. He became a hero to conservatives by triumphing over public employee unions in a high-profile battle over collective bargaining rights, then survived a recall election. But Mr. Walker also has the record, experience and temperament of a candidate who could attract significant support from the party establishment. He is not a Ted Cruz or a Rick Santorum, even though he promises to appeal to many of the same voters.

Mr. Walker has won three elections for governor in a state that has voted Democratic in every presidential election in the last 30 years. He has earned the support of several experienced Republican operatives, including alumni from the Republican National Committee. He seems to be raising substantial amounts of money. He surged to the lead in polls of Iowa, and he is tied in national polls as well, even though many Republicans still don’t know enough about him to formulate an opinion about him.

Whether Mr. Walker can win the nomination will greatly depend on how he performs in the visible primary: the public exhibitions and competitions, like debates, interviews and speeches. Compared with Mr. Bush or even Mr. Rubio, Mr. Walker is an unfamiliar candidate. He has not auditioned on the national stage. Many donors and officials might adopt a wait-and-see approach, ultimately rallying behind him if he proves to be as strong on the campaign trail as he is on paper. Something similar happened to Mr. Obama, who received only a few endorsements before winning the Iowa caucuses.

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Mr. Rubio, in contrast, is a more familiar candidate. Fewer donors and operatives will have serious questions about his ability to compete in national politics. But unlike Mr. Walker, Mr. Rubio is not well positioned against Mr. Bush. Mr. Rubio is acceptable to conservative voters who make up the natural opposition to Mr. Bush, but he will have a hard time becoming their first choice. His support for immigration reform cost him among the conservative rank and file, who might prefer factional candidates like Mr. Cruz or Mike Huckabee, even if Mr. Walker faltered. They might even ultimately prefer candidates like Mr. Jindal or Mr. Perry.

Mr. Rubio’s preferred path to the nomination involves keeping Mr. Bush from winning the invisible primary by a wide margin, and outmuscling Mr. Bush for many of the same people who might have otherwise supported him. It’s not impossible to imagine: Mr. Rubio is a talented speaker with an appealing biography. He is a young candidate with a new brand, and he would appeal to those who don’t want another Bush. He has also spent much of the last four years cultivating the party’s Washington establishment.

But if Mr. Rubio can’t keep Mr. Bush out of the category of invisible primary winner, his path to the nomination starts looking quite narrow.