Michigan Democratic Sen. Debbie Stabenow's lead over one of her leading Republican challengers has widened over the past six months, but her approval ratings have slipped further under water.

The two-term incumbent now leads former Rep. Pete Hoekstra by nine points, 47 percent to 38 percent, according to a survey conducted by the Lansing-based firm EPIC-MRA. In February, months before Hoekstra announced his candidacy, Stabenow held a narrow two-point lead.

The senator has solid support from her Democratic base, with 85 percent backing her, and she holds an 18-point lead among independent voters. Hoekstra, who lost his party's nomination for governor last year, attracts 79 percent support from his base.

The majority of respondents -- 54 percent -- say Michigan's economy is headed in the wrong direction. However, no matter how voters view the economy, they favor Stabenow over Hoekstra. Among those who say the economy is getting better, she leads by 49 percent to 37 percent; among those who say it's not getting better, she leads by a slightly larger margin, 50 percent to 38 percent; and among those who think it will get worse, she still leads, 43 percent to 38 percent.