BOULDER, Colo. -- You'd think it was a concert, just looking at the crowd.

But instead it was a register to vote rally, headlined by Michelle Obama, wife of the Democratic nominee for president.

Thousands of University of Colorado students packed an open field Wednesday morning to hear Obama encourage young people to register to vote.

The rally started at 11:15 a.m. but some students were already in line at 7 a.m. CU's fire chief estimated the crowd at Farrand Field, a sprawling lawn surrounded by dormitories, to be 9,700 people, the largest crowd ever for the field, according to CU.

Workers with clipboards signed up people to vote as they waited to get in.

Obama said her husband, Sen. Barack Obama, is the only candidate who understands the struggles of the middle class. She said it was only a few years ago that he finished paying his student loans. She told the crowd they play an important role in deciding who becomes president and underscored the importance of Colorado in the election.

Before Obama took the stage, Jared Polis, who is running for Congress, and the wife of Senate candidate Rep. Mark Udall told students to make sure that they talk to friends, family, and others in their social networks about voting. Former Bronco Rod Smith energized the crowd with his presence and then admitted he hadn't vote in the last five presidential elections.

"I suffered enough by not voting and you know what I did? I complained ... If you don't vote, don't complain," Smith said.

He said he voted in the last election and is now actively engaged in the political process, stumping for Barack Obama after they met in February.

Wednesday's rally was hosted by the University of Colorado Student Union and sponsored by the Boulder campus chapter of the College Democrats.

Michelle Obama's visit comes two days after her husband visited a Westminster high school.

John McCain is also set to visit Denver and Pueblo Thursday and Friday. Running mate Sarah Palin will attend a private fundraiser on Centennial on Saturday.