RACINE, Wis. -- U.S. first lady Michelle Obama's favourability ratings are higher than her husband's, and the campaign is sending her to critical battleground states to help Barack Obama regain the lead that has slipped in the final two weeks before Election Day.

"You know what this room feels like? It feels like four more years in here!" Mrs. Obama exclaimed as a crowd of 2,500 in Wisconsin roared its approval. Some had waited overnight to see her.

Mrs. Obama can make the case for her husband's re-election over Republican Mitt Romney in a way no one else can, said Anita McBride, who served as Laura Bush's chief of staff in the White House.

"The first lady is always good for a couple of important things," McBride said. "One is the human side of the president, seeing who they are personally through the lens of the spouse, which is always unique."

One of Mrs. Obama's favourite parts of the campaign is being able to praise her husband without him hearing, she told the Wisconsin crowd: "So don't tell him how much I really love him. I kind of keep that leverage. That's between you and me."

McBride called Michelle Obama a skilled campaigner who is much more enthusiastic this time around than in 2008, when she initially appeared reluctant to speak out on behalf of her husband.

"She never loved the political game, and she was honest about that" in 2008, McBride said. But this year, "she is comfortable on the stump. She brings a lot of energy and she is in it to win it."

The first lady's campaign speech mixes the personal and political, offering glimpses of life in the White House and some of the reasons she married the president, along with an impassioned defence of his policies and character.

She beseeches crowds to get out and vote and urge friends and family to do the same.

"This election will be closer than the last one. That's the only guarantee," Mrs. Obama said.

The first lady heads to Hollywood on Thursday to appear on Jimmy Kimmel's late-night TV show and raise money with actor Will Smith.

Jeremy Bird, national field director for the Obama campaign, said Michelle Obama is a valuable "grass-roots champion."