Last week, Mitt Romney. This week, Michelle Obama.

The first lady made a campaign stop in Philadelphia Wednesday, speaking to a crowd of about a thousand supporters at the National Constitution Center on Independence Mall.

It was a brief stop in the city, just a 20-minute speech -- no fundraisers or factory tours.

This trip was not about persuasion. It was about inspiring the Obama faithful in a critical city of a critical state.

President Obama needs a robust turnout in Philadelphia to carry Pennsylvania, so Michelle Obama came to reward volunteers with a special moment, and try and keep them motivated.

Tickets to the event were free, but were available through Obama campaign field offices in city.

"Don’t forget this election will be even closer than ever before," Obama told the crowd. "Remember that in the end, this could all come down to those last few thousand people that we register to vote ... to those last few thousand people we get to the polls on Nov. 6."

In recent weeks some political analysts and even the state Republican chairman have questioned whether Pennsylvania is really in play -- whether Mitt Romney has a chance here.

But Republicans haven’t give up. The political group associated with GOP operative Karl Rove, Crossroads GPS just bought about $700,000 worth of TV time in the state to start running another hard-hitting ad attacking the president. 

The ad includes 2008 footage of Barack Obama saying, "we can’t mortgage our children’s future on a mountain of debt." An announcer then says, "Now he’s adding $4 billion in debt every day, borrowing from China for his spending.”

Michelle Obama told supporters at the Constitution Center that undecideds are important, and they should remember the campaign’s message.

"You can remind folks that Barack kept his promise to bring our troops back from Iraq," she said. "And you can remind them about how our brave men and women brought to justice the man behind the 9/11 attacks. You remind them about that."

The first lady finished with a call to action.

"I have one last question to ask you. Are you in?" she said, prompting enthusiastic cheers.

The room was full of the kind of energy often apparent in the closing days of a campaign. Obama's challenge will be keeping supporters fired up for another five months.