The outcome could be critical to Republican efforts to get a majority in the U.S. Senate. The incumbent, Democrat Tom Harkin, is retiring after five terms.
The Quinnipiac University Poll showed Braley, now serving in the U.S. House, with 44 percent of the vote to 40 percent for state Sen. Joni Ernst. In March, Braley was ahead 42 percent to 29 percent.
Much of the change appears to come from Ernst's greater name recognition. In March, 80 percent of respondents said they did not know enough about her to have any opinion, which has dropped to 37 percent.
"Now she is as well-known as Bruce Braley, and his double-digit lead has all but evaporated," said Peter A. Brown, the poll's assistant director.
In what Brown called an "unusual split," Braley led by 11 percentage points among female respondents. But 49 percent of respondents said Ernst would do a better job on women's issues while 30 percent said Braley would.
The candidates were close on most individual qualities the poll looked at. For example, 43 percent of voters said Braley is "about right" on the political spectrum, while 26 percent called him too liberal. For Ernst, 41 percent found her "about right" and say he is too liberal. Ernst is "about right," 27 percent too conservative.
Quinnipiac surveyed 1,277 registered Iowa voters between June 12 and June 16. The margin of error is 2.7 points.