MADISON, Wis. (AP) -- Democrat Barack Obama continues to have a double-digit lead over Republican John McCain in Wisconsin, an independent poll released Thursday showed.

Obama has an 11-point lead over McCain in the Quinnipiac University poll, down from 13-points in a June poll. The two-point drop this month was within the poll's 3-point margin of error and statistically insignificant.

McCain made clear advances in three other states polled -- Minnesota, Michigan and Colorado -- so they are now too close to declare a leader. In Minnesota, McCain narrowed a 17-point Obama lead to just 2 points.

But Obama's lead remained solid in Wisconsin, with 50 percent of likely voters saying they would vote for the Illinois senator while just 39 percent said they back McCain, a senator from Arizona.

"Sen. Barack Obama's post-primary bubble hasn't burst, but it is leaking a bit," said Peter Brown, assistant director of the Quinnipiac University Polling Institute.

The McCain and Obama campaigns had no immediate comment on the numbers.

Brown said one possible reason for McCain's improvement in states other than Wisconsin is the energy issue. The poll showed increased support for additional oil drilling in the U.S., which McCain backs but Obama opposes.

In Wisconsin, 52 percent said the candidate's energy policy is more important than his policy on the Iraq war. Thirty-four percent said Obama has the better energy policy, compared with 29 percent for McCain.

Brown said Wisconsin remains "the most liberal of swing states, giving Obama a comfortable double-digit lead."

The poll of 1,094 people in Wisconsin was conducted between July 14 and Tuesday.

Two more polls by the Connecticut university are to be done between now and the Nov. 4 election. The poll was done in conjunction with The Wall Street Journal and The Washington Post.

Wisconsin voters narrowly went for Democratic presidential candidates in both 2000 and 2004, putting it on a short list of states viewed as winnable by both sides. Still, McCain branded himself as the underdog during a stop in the state earlier this month.

Both candidates have been running television ads here. On Thursday, Obama's campaign planned a fundraiser in Madison, while Republican National Committee Chairman Mike Duncan planned to visit with McCain workers in the West Allis office.

(Copyright 2008 by The Associated Press. All Rights Reserved.)