CNN reported that Obama, the junior senator from Illinois, has expanded his base of younger, college-educated voters, and has eroded Clinton's base of working-class Americans.

Among white seniors, Obama's support increased by 8 points in Wisconsin, from 31 percent to 39 percent since Super Tuesday, CNN reported.

Obama has also won almost half of the Catholic vote in Wisconsin on Tuesday, compared to a third of it in polls two weeks ago.

The next Democratic nominating contests will be in Ohio and Texas March 4.

Clinton, the New York senator who has lost 10 straight post-Super Tuesday nominating contests to Obama, is banking on a win in Texas, where she has enjoyed a high degree of support from Hispanics.

But polls in Texas show Obama and Clinton in a virtual tie in the Lone Star State, CNN reported.

Obama and Clinton will debate Thursday in Houston.