The Republican polling firm Rasmussen conducted a one-day survey of 500 “likely voters” in Iowa on March 17. Click here for topline results.

Senator Chuck Grassley still leads all his Democratic challengers, with no statistically significant change in his lead since Rasmussen’s last Iowa poll in February. He leads Roxanne Conlin 55 percent to 36 percent (the February numbers were 53-36). Grassley leads Bob Krause 57-31 (55-33 in February), and he leads Tom Fiegen 57-28 (56-28 in February).

Instead of asking respondents whether they approved of Grassley’s work in the Senate, Rasmussen asked whether they had a favorable or unfavorable impression of the senator. He was at 66 percent very or somewhat favorable, 31 percent very or somewhat unfavorable. (The latest Selzer poll for the Des Moines Register measured Grassley’s approval at 54 percent, but favorability numbers can often run ahead of approval numbers.)

Clearly the Senate race is still Grassley’s to lose, but he’s not likely to be re-elected with the huge margins he’s had in the past. There is also plenty of time for the race to tighten up if Grassley makes big mistakes. As Senate Guru reminded us in this diary, the current fundraising quarter ends March 31. I encourage Democrats to get involved and support one of Grassley’s challengers. Here are links to donate to the Conlin campaign, the Fiegen campaign and the Krause campaign.

Other notable findings from the latest Rasmussen poll: President Barack Obama’s approve/disapprove numbers were 50/49, but Governor Chet Culver is still in negative territory at 41 percent approve/57 disapprove. About 45 percent of respondents said they favored “the health care reform plan proposed by President Obama and the congressional Democrats,” while 53 percent said they opposed it. Remember, this poll was in the field before Congress gave final approval to the bill Obama signed yesterday. I am curious to see future polling on the issue. A quickie USA Today/Gallup nationwide poll released yesterday was the first in a long time to show net positive approval for health care reform: 49 percent of respondents said it was a “good thing” that Congress passed the bill over the weekend, while 40 percent said it was a “bad thing.”