A new Marquette University poll finds President Obama and Mitt Romney are essentially tied among likely voters in Wisconsin, at 49 percent to 48 percent. 

That's a significant shift in Romney's favor. Two weeks ago, before the first presidential debate, Obama held an 11-point lead in Marquette polling. “Rarely has a debate produced such a large movement in the polls. In September, President Obama held a steady lead, but now the race is a pure tossup, in large measure because of the first debate,” said Marquette Law School Poll Director Charles Franklin.

The poll was taken October 11 to 14; we'll have to wait for the next one to see if the second debate had any effect.

An NYT/CBS/Quinnipiac poll taken last week also found a tighter race in Wisconsin after the first debate. In that survey, Obama had a three-point edge among likely voters 50 percent to 47 percent.

Wisconsin is shaping up to be a critical battleground in this election, with a large chunk of persuadable voters. No other state has as many counties that voted for both George W. Bush and Obama.