Click here to listen to "Fighter."

Going into the Pennsylvania primary with a sizable lead but depleted coffers, Hillary Rodham Clinton's campaign decided early on to forgo the strategy of expensive statewide ad buys it had used in previous contests in favor of niche advertising and grassroots organizing. With Clinton's funds still low, state polls tightening and Barack Obama having outspent her 5-to-1 in advertising, a pro-Clinton union has decided to supplement her efforts with an ad buy of its own.

The American Federation of Teachers on Monday released a radio spot (subscription) in markets across the state, touting Clinton's "solutions" to the country's economic problems and knocking Republican John McCain as "another George Bush." (It makes no mention of Obama.) The group has previously run ads supporting Clinton in Iowa, New Hampshire and other primary states, and has now invested about $2 million in media buys on her behalf.

The group's newest radio spot features the voices of Pennsylvania citizens lauding Clinton for her ability to solve problems created by President Bush and supported by McCain. Throughout, the emphasis is firmly on Clinton's toughness, and the ad repeatedly refers to her as a "fighter" -- a key Clinton talking point throughout the campaign and one that could be especially effective in a state that some local political consultants say rewards a tougher brand of politics. "When she gets knocked down, she gets up and keeps fighting," says one woman featured in the ad.

The AFT spot joins just one Clinton ad currently running on Pennsylvania airwaves, a reworked version of a populist ad her campaign ran previously in Texas and Mississippi. In contrast, Obama has not only produced new ads for the Keystone State but has expanded his paid media campaign into Indiana and North Carolina, where Clinton has yet to invest any ad dollars. Until Clinton can close the cash gap separating her campaign from Obama's, she will be forced to rely on her ground game rather than taking the fight to the airwaves.