Jeb Bush, the early establishment favorite for the Republican nomination, has suggested that he doesn’t want to have to pander to the party’s base to win the nomination.

He’s probably going to need to.

A Bloomberg Politics/Des Moines Register poll painted an abysmal picture of Mr. Bush’s standing among the party’s Iowa caucus-goers: a well-informed microcosm of the conservative wing of the party, and perhaps a leading indicator of how conservatives will react as the primary season unfolds.

Since October, Mr. Bush’s unfavorable rating has increased from 28 to 43 percent among likely Iowa caucus voters. His favorability rating even dipped by four points, to a tepid 46 percent. Thirty-seven percent believe he is too moderate.

Mr. Bush has run well in the behind-the-scenes competition for elite support over the last few months. The strength of that support — among donors, party officials, elected officials and campaign operatives — was a central reason Mitt Romney decided to forgo a campaign. But for all of Mr. Bush’s strength among the party’s elite, he has never held much support among the G.O.P. rank and file.