DES MOINES — U.S. Rep. Bruce Braley has extended his lead over the Republican field for an open seat in the U.S. Senate.

Braley has a double-digit lead — 12 to 15 percentage points — over most of the largely unknown GOP hopefuls for the seat of Sen. Tom Harkin, who is retiring after five terms, according to a Quinnipiac University Poll released Thursday morning.

Although his margins have improved, Braley has yet to break 50 percent in the Quinnipiac poll. He leads his nearest possible competitor, former energy company CEO Mark Jacobs, 40 percent to 31 percent in the poll of 1,411 registered voters from March 5-10.

That’s an improvement for Braley from a Quinnipiac poll in December that showed the four-term Democrat representative’s lead over former U.S. Attorney Matt Whitaker to be just 3 percentage points — 43-40 — or barely more than the poll’s margin of error.

At the time, Peter Brown, assistant director of the Quinnipiac poll, attributed Braley’s showing to President Barack Obama’s unpopularity.

Obama still is unpopular among Iowans, according to Quinnipiac. The president gets a negative 39 percent to 57 percent approval rating from Iowans, roughly the same as his 38 percent to 59 percent disapproval in December.

In a Quinnipiac poll last summer, 29 percent said their vote in the U.S. Senate race would be a vote against Obama, while 14 percent said it is a vote for Obama.

In the most recent poll, Braley gets a 35 percent to 18 percent favorability rating, while 46 percent of Iowa voters don’t know enough about him to form an opinion.

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“The curse of President Obama’s low approval does not seem to be hurting U.S. Rep. Braley,” Brown said.

Or it might be that 77 percent to 85 percent of voters don’t know enough about the GOP candidates to form an opinion.

So nearly eight months out from the general election, Braley holds a nine-point lead over Jacobs, who has advertised on radio and TV more heavily than his GOP rivals, and leads Whitaker 42 percent to 30 percent, state Sen. Joni Ernst 42 percent to 29 percent and college professor Sam Clovis 42 percent to 27 percent.

The poll, which has a margin of error of plus or minus 2.6 percentage points, was conducted by live interviewers calling land lines and cellphones.

Quinnipiac also found that Iowa voters approve of the job U.S. Republican Sen. Chuck Grassley is doing by a 62 percent to 27 percent margin and approve of Harkin’s performance 55 percent to 31 percent.