The Suffolk University Political Research Center asked 800 Iowa "likely voters" about this year's biggest races. The margin of error for the survey, conducted between April 3 and April 8, is plus or minus 3.5 percent. Suffolk's press release summarizing the highlights is here. Full results are here (pdf). Tables are here (pdf).
Representative Bruce Braley leads all Republican rivals for U.S. Senate in the first Iowa poll conducted after Braley's comments about Senator Chuck Grassley gained wide attention. Braley is still better-known than the GOP candidates, and more Iowans have a favorable than unfavorable impression of him. The bad news for Braley is that he is below 40 percent against each of the Republican candidates.
Suffolk's poll indicates that the GOP IA-Sen primary is now a two-tier race, with State Senator Joni Ernst and Mark Jacobs each commanding more than 20 percent support, and the other candidates in the single digits. That makes sense, since Ernst and Jacobs have the most establishment support and are the only Senate candidates who have been able to raise their name recognition through paid advertising. But 40 percent of respondents were undecided.
Governor Terry Branstad's still in positive territory, with 48.5 percent of respondents viewing him favorably and about 35.4 percent unfavorably. His lead over Democratic State Senator Jack Hatch is smaller in this poll than in any other Iowa survey I've seen, though: 42.4 percent to 32.1 percent.
Among respondents who said they are likely to participate in the 2016 Democratic caucuses, 63 percent favor Hillary Clinton. U.S. Senator Elizabeth Warren was far behind with 12 percent, followed by Vice President Joe Biden with 10 percent. It's hard to say who is really in second place, since the margin of error for the Democratic caucus-goer subsample is quite large (plus or minus 8.4 percent). Nevertheless, Clinton clearly maintains a commanding lead.
I wouldn't read much into the Iowa GOP caucus results from this survey. All the potential presidential candidates (Mike Huckabee, Rick Santorum, Rand Paul, Jeb Bush, Ben Carson, Chris Christie, Paul Ryan, Scott Walker, Sarah Palin, Marco Rubio, and Condoleezza Rice) are clumped close together, between 6 and 11 percent support. That's within the the margin of error of plus or minus 8.7 percent for that subset of the Suffolk poll.