(MCCLATCHYDC) — Republican Joni Ernst has opened up a 6 percentage point lead over Democrat Bruce Braley in Iowa’s closely watched U.S. Senate race, according to a new Des Moines Register Iowa poll.

“The ground under Bruce Braley has shifted,” a Register analysis said.

The race to replace retiring Sen. Tom Harkin, D-Iowa, has been regarded as too close to call. But the new survey, taken Sept. 21-24, found Ernst up 44 to 38 percent. Braley, a congressman, can still catch up, as 12 percent remain undecided. Republicans need a net gain of six Senate seats this year to win control of the Senate.

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