MADISON, Wis. -

Wisconsin's tightening race for the U.S. Senate may signal a shift to the left in a state that has been leaning to the right ever since Republicans scored significant gains following the 2010 election.

A new Marquette University Law School poll released Wednesday shows Democrat Tammy Baldwin with a narrow 4-point lead over Republican Tommy Thompson. The poll of 894 likely voters done Sept. 27 through Sunday has a margin of error of plus or minus 3.3 percentage points.

Baldwin's lead is down from 9 points two weeks ago.

Marquette pollster Charles Franklin said Baldwin's lead dropped due to a shift in support from independent voters.

The poll also shows President Barack Obama with an 11-point lead over Republican Mitt Romney.

The latest Marquette University Law School poll shows Obama leading Romney 53-42 percent.

But the president's lead has narrowed slightly in the past two weeks. In a poll conducted in mid-September, Obama led by 14 points among likely voters with a margin of error of plus or minus 4 percentage points.

The Romney campaign has said the polls over-sample Democrats and believes the presidential debate Wednesday night could change the game.

"Debates can have an impact. They can reach the voters who are undecided and tuning in," Franklin said.

But Franklin said his poll indicates there are few undecided voters left. He said only 2 percent of those polled said they were very likely to change their minds.

"We've found that, first of all, voters said they wanted more details from both candidates, but asked if they would change their minds, only 2 percent said they were very likely to, 66 percent said they weren't likely at all to and the rest were sort of in the middle. So put it all together and probably we've got maybe 15 percent who say they're at least willing to consider changing their minds," Franklin said.

Obama plans to campaign in Madison on Thursday.