Democratic Senate candidate Roxanne Conlin and Governor Chet Culver markedly improved their position in the latest statewide poll by Research 2000 for KCCI-TV. The pollster surveyed 600 likely Iowa voters between May 3 and May 5, producing a margin of error of plus or minus 4 percent.

In the Senate race, five-term incumbent Republican Chuck Grassley leads Conlin 49 percent to 40 percent. The last time Research 2000 polled this race for KCCI in mid-February, Grassley led Conlin 56-35. The firm has not polled Grassley against either of the other Democratic Senate candidates, Tom Fiegen and Bob Krause.

To my knowledge, Grassley has never been below 50 percent in a public poll before. The favorability numbers suggest that support for Conlin has more room to grow, because 20 percent of respondents didn’t know enough about her to have an opinion. Only 5 percent of respondents said the same about Grassley. Michael O’Brien of The Hill declared Conlin “within striking distance” of Grassley.

In the governor’s race, the new poll found former Governor Terry Branstad leading Culver 48 percent to 41 percent. Normally those numbers wouldn’t look good for an incumbent, but in Research 2000’s February poll for KCCI, Branstad’s lead was twice as large (54-38). DavidNYC of Swing State Project quipped that Culver’s numbers no longer resemble those of the 1962 New York Mets but look more like those of the 1963 Mets. Culver led Bob Vander Plaats 44 percent to 40 percent and Rod Roberts by 46 percent to 36 percent.

I frankly expected worse numbers in this poll. The three Republican candidates have been criss-crossing the state bashing Culver full-time for months now. Branstad, Vander Plaats and Roberts have held two debates and countless campaign events and media interviews in towns large and small. Furthermore, Branstad has been running paid television advertising statewide for a full month. Culver’s campaign manager Donn Stanley emphasized that angle in his comment on the poll: “What is particularly surprising is that this poll comes out after weeks of Branstad’s campaign airing hundreds of thousands of dollars worth of television ads across Iowa. He is the only candidate in the race that is running television ads. This poll suggests those ads have not be resonating with Iowa voters.”

Branstad’s campaign spokesman Tim Albrecht told KCCI, “Polls will go up and down, but what’s unchanged is that Governor Branstad is the Republican who can beat Chet Culver in November.”

One problem with the poll is the partisan makeup of the sample: 33 percent Democrats, 29 percent Republicans and 38 percent Independents. That’s quite different from the proportion of Iowans who cast votes in the 2006 general election (pdf file available here): 37 percent were Democrats, 37 percent were Republicans, and 26 percent independents. I would be very surprised if the voter universe this November had a plurality of no-party voters.

Both Grassley and Branstad led comfortably among no-party voters in the new KCCI poll, so if that poll over-sampled independents, the Republican leads in the Senate and governor’s race might be even smaller than they appear. On the other hand, there’s no guarantee that this November’s voter universe will contain more Democrats than Republicans, as this poll assumes. Iowa Democrats still have a voter registration advantage of about 100,000 over the GOP, but Republicans may benefit from an “enthusiasm gap.”

What do you think, Bleeding Heartland readers?

UPDATE: Secretary of State Michael Mauro released the latest Iowa voter registration numbers today: 602,768 Republicans, 711,106 Democrats, and 774,005 no-party voters. The total number of registered voters is 2,089,561. Approximately 1,050,000 Iowans voted in the 2006 general election.