Obama still has clear advantages in the state, which hasn't voted Republican in a presidential race since Ronald Reagan in 1984. He's got a field organization that Democrats say is stronger than ever, regional appeal as a neighbor from next-door Illinois and an electorate receptive to his message of change.
But McCain has gained ground on Obama through a barrage of television ads, campaign events and the selection ofas his running mate. He's also sharply increasing the number of campaign aides and offices he's got in the state to counter Obama's organizational strength.
The changing dynamic has emerged as the Republican Party base, which had been lukewarm to McCain, has been energized by the addition of Palin to the ticket. Across the state, Republicans describe a wave of new volunteers and contributions after the Alaska governor joined.
"They're saying it grabbed their attention and changed their stance about standing on the sidelines and getting involved," said Tom Van Drasek, chairman of the Brown County Republican Party in Green Bay. "Now the energy level is way up."
Democrats concede that it now looks like a closer general election for Wisconsin's 10 electoral votes than they'd originally expected.
"It makes the chance of a bigger Obama victory much less," said Paul Maslin, a Democratic pollster based in Madison. "I still think Obama's the favorite in this state, but I don't think anybody can be overconfident. It's still probably going to be fairly close."
Democratic candidates Al Gore and John Kerry each narrowly carried Wisconsin over George W. Bush. Kerry's 11,000-vote victory in 2004 made Wisconsin the state with the narrowest margin of victory at less than 0.4 percent.
McCain has dubbed himself the underdog in the state. But even before he selected Palin, he had succeeded in cutting into an Obama lead that polls showed was as high as 13 points in June. McCain did so by visiting the state over the summer while Obama campaigned elsewhere and by outspending Obama on ads questioning his opponent's readiness for the presidency.
McCain's campaign and the Republican Party spent nearly $2 million on ads in Wisconsin between June 3 and July 26, nearly twice as much as the Obama campaign, according to the latest figures available from the Wisconsin Advertising Project.
Now McCain is working to catch up with what Democrats say is Obama's biggest strength in the state; organization; by increasing the number of campaign offices from 10 to 18 and paid staff members from 20 to about 30.
Obama's campaign has 43 offices. Dozens of paid staff members, the campaign won't say precisely how many are working to identify supporters.
State Republican Party Chairman Reince Priebus wouldn't say whether he thinks McCain still is an underdog.
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"Certainly we have our work cut out for us. But at this point, all of the energy is in our direction," Priebus said. "I think it's neck and neck."
After a slow start, the Obama campaign has stepped up its activity in Wisconsin in recent days. Obama campaigned in Eau Claire and Milwaukee in the last two weeks, and running matemade his first visit to Wisconsin last week, courting Packers fans in Green Bay. McCain and Palin will pay their own visit to Green Bay on Thursday.
Wisconsin Democratic Party Chairman Joe Wineke said the Obama campaign "has the most sophisticated and intense ground game in this state I've ever seen" in the last 30 years. The campaign has at least twice as many offices as Kerry had open in 2004, Wineke said, some in places where Democrats rarely have spent time.
In addition, some 155,000 Wisconsin residents have registered to vote for the first time since the beginning of the year, nearly a third of them in Democratic-leaning Milwaukee County.
The McCain campaign's decision to add offices and campaign workers points to a renewed belief that Wisconsin is winnable for McCain.
Republicans say much of the enthusiasm revolves around Palin, whose anti-abortion views have excited social conservatives who were lukewarm on McCain.
"It's always nice when you feel the energy kick in," said Republican Party executive director Mark Jefferson. "This has been a real shot in the arm to those people who did think maybe this wasn't their year."