Wisconsin Governor Scott Walker has expanded his early lead in Iowa, while former Florida Governor Jeb Bush continues to face headwinds and Senator Marco Rubio of Florida shows upside potential in the state that hosts the first 2016 presidential nomination balloting.

A new Bloomberg Politics/Des Moines Register Iowa Poll shows more than a third of likely Republican caucus participants say they would never vote for Bush—one factor in a new index to assess candidate strength in such a crowded field. Forty-three percent view him favorably, compared to 45 percent who view him unfavorably.

Read the poll questions and methodology here.

Walker is backed by 17 percent as the state enters a busy summer of candidate visits, a planned straw poll, and campaigning at the Iowa State Fair. Tied for second are Senator Rand Paul of Kentucky and retired neurosurgeon Ben Carson at 10 percent, with Bush and former Arkansas Governor Mike Huckabee next at 9 percent each.

They’re followed at 6 percent by Rubio and 2012 Iowa caucuses winner Rick Santorum, a former senator from Pennsylvania. With eight months to go before the 2016 caucuses, there’s plenty of time for movement.

“Scott Walker’s momentum puts him solidly in first place,” said J. Ann Selzer, president of West Des Moines-based Selzer & Co., which conducted the poll. “For the time being, he’s doing the right things to make the right first impression.”

In the previous Iowa Poll, taken in January, Walker stood atop the field at 15 percent.

Walker shouldn’t count on an Iowa win just yet, especially with such a large and unsettled field. Four years ago, former Massachusetts Governor Mitt Romney and then-Minnesota Representative Michele Bachmann sat atop the Iowa Poll at 23 percent and 22 percent. Romney ultimately finished a close second in Iowa, while Bachmann ended up sixth and exited the race the following morning.

Read the article “Scott Walker Has Early Lead in Iowa” on bloomberg.com.