"Don't boo, vote," President Obama often says in his stump speech whenever crowds boo a Romney plan.

The off-hand call to vote may be by design. It comes amid a precipitous decline in Democratic voter registration in key swing states -- nowhere more apparent than in Ohio.

Voter registration in the Buckeye State is down by 490,000 people from four years ago. Of that reduction, 44 percent is in Cleveland and surrounding Cuyahoga County, where Democrats outnumber Republicans more than two to one.

"I think what we're seeing is a lot of spin and hype on the part of the Obama campaign to try to make it appear that they're going to cruise to victory in Ohio," Cuyahoga County Republican Chairman Rob Frost said. "It's not just Cuyahoga County. Nearly 350,000 of those voters are the decrease in the rolls in the three largest counties, Cuyahoga, Hamilton and Franklin."

Frost points out that those three counties all contain urban centers, where the largest Democrat vote traditionally has been.

Ohio is not alone. An August study by the left-leaning think tank Third Way showed that the Democratic voter registration decline in eight key swing states outnumbered the Republican decline by a 10-to-one ratio. In Florida, Democratic registration is down 4.9 percent, in Iowa down 9.5 percent. And in New Hampshire, it's down down 19.7 percent.

"It's understandable that enthusiasm is going to wane a little bit from that historic moment (in 2008)," says Michelle Diggles, the study co-author and senior policy adviser for Third Way. "You can only elect the first African-American president of this country once."

The dip in registration has been framed by some as the result of Republican efforts to suppress the vote - an accusation that Ohio Secretary of State John Husted, a Republican, categorically rejects.

"That's kind of a silly notion that removing deceased people and duplicate records from the roll has anything to do with voter suppression,” he said. "It actually has to do with voter integrity. They can't point to one legally registered voter that's actually been removed from the rolls."

The Third Way study, which was conducted in August, indicates the Democrats' drop in registered voters coincides with a gain in independent voters.

"There are about half a million more independents now than there were just for years ago," Diggles said.

One Democratic Party consultant told Fox News that independents in Ohio may be leaning Democratic - an effect that may be tied to the bailout of Chrysler and GM. One of eight people in Ohio work in businesses directly tied to the auto industry. The state has been carpeted with Obama ads that point to his bailout of the industry and it's managed bankruptcy.

Mitt Romney also favored a managed bankruptcy of the auto industry. But he criticized the expenditure of taxpayer money and the preferential treatment given to union-linked creditors over the industry's secured creditors.

Others question the bailout's effect on swaying the minds of independent voters. In the words of Diggles, independents are "not a stable voting block at all."