The Republican primary battle has been called a wide-open contest — the “deepest” and “strongest” field in many cycles. It is full of the party’s rising stars and hopefuls, like Scott Walker, Marco Rubio, Rand Paul, Chris Christie, Ben Carson, Ted Cruz and Bobby Jindal, alongside the most formidable name in Republican politics: Bush.

But the campaign is not nearly as open as it looks. Many of the candidates who have received the most news media attention have little or no chance of winning the nomination. Instead, two figures — Jeb Bush and Mr. Walker — have quickly moved to the head of the pack. Perhaps only Mr. Rubio has a good chance to join them at the top.

Mr. Bush and Mr. Walker could implode, of course, with a Rick Perry-esque “oops” or an unexpected scandal. But if they avoid such mistakes they will be difficult to dislodge, even with strong fund-raising by rivals like Mr. Cruz.