Indeed, the Wisconsin recall could signal a GOP shift in an arc of states from Iowa to Pennsylvania that have reliably voted Democratic in presidential elections for a generation.

"Whether Walker wins or doesn't is going to be a big indicator of how Wisconsin goes, and how the whole upper Midwest goes," said Iowa's Republican Gov. Terry Branstad.

Romney has signaled plans to contest Iowa, where Obama's 2008 caucus win propelled him to the Democratic nomination. Romney also sees opportunity in his native Michigan, where Democratic presidential candidates have won since 1988.

Bright spots are developing for Obama, too.

Public polls this month showed the president narrowly ahead in Virginia, a Southern state Republicans had carried nine times before Obama won it in 2008. Obama's advantage among Latino voters is moving New Mexico his way. Neither campaign nor the super PACs have advertised there, despite close finishes in 2000 and 2004.

Obama also has seized on new economic data that could give him a lift across the contested map. April unemployment ticked downward in all of the up-for-grabs states except Colorado as Obama and Romney have fought over who is best equipped to lead an economic recovery.

In Des Moines, Iowa, this month, Romney blamed Obama's spending for the recovery's slow pace. A week later, on the other side of town, Obama said Romney's career as a private equity executive was more suited for the boardroom than the Oval Office.

Obama's attack dovetails with scathing ads on Romney's career at the head of Bain Capital, which ran briefly in Colorado, Iowa, Pennsylvania and Virginia. They remained on the air last week in Ohio, where Obama aides say Romney's opposition to the auto industry bailout in 2009 hurts him with workers in the region's auto manufacturing sector.

Obama has had an edge in getting out his message. For nearly two months, his campaign has aired spots across 11 states, heaviest in Florida, Iowa, Ohio and Virginia, according to the ad-tracking reports.

Romney has only been airing ads for two weeks in four states. But super PACs that support him have helped shave Obama's advertising edge, airing $10 million in ads across 10 states.

Obama aides point to an edge in state-by-state organizing that could be the deciding factor in a close election. While Romney is quickly arranging with the Republican National Committee to deploy staff to various battlegrounds, Obama's campaign has been up and running for years.

Said Democratic strategist Devine: "The president and his campaign have a real and potentially decisive advantage on the ground."