Jeb Bush had a good month. He struck first in the invisible primary — the behind-the-scenes competition for the support of donors, officials and operatives. And for all of the talk about the depth of the Republican field, the opposition to Mr. Bush remains unclear and underwhelming.

That is in part because many of the high-profile potential Republican contenders — like Ted Cruz, Chris Christie and Rand Paul — are factional and flawed candidates with tough routes to the party’s nomination.

Nonetheless, there will be demand for an alternative to Mr. Bush, even from within the so-called Republican establishment. Since Friday, attention has focused on Mitt Romney, who said in a meeting of top advisers and donors that he was considering a third run.

But the more compelling challenger may be Scott Walker, the battle-hardened governor of Wisconsin. He has made moves toward running, and on paper, he’s the type of candidate who should deeply concern Mr. Bush.