In the hunt for 270, Obama starts with more states and electoral votes in his column. Romney must take back from the incumbent some states that Obama carried four years ago, including North Carolina and Virginia, which had been reliably Republican until 2008.
In Virginia, public and private polls show Obama narrowly ahead. Internal Republican polls have shown Romney leading in Loudoun and Prince William counties. Over time, these once reliably Republican counties have become more politically diverse, as younger, well-educated, racially and ethnically diverse voters have flocked to Washington's suburbs. Obama won four years ago by aggressively going after them and the state's robust African-American electorate.
Romney can win Virginia by taking Loudoun County away from the Democrats, holding down Obama's likely edge in other Washington suburbs, and running up big numbers in rural southern Virginia and the conservative Tidewater area.
In Ohio, Romney needs Hamilton County, especially the Cincinnati suburbs, to offset Obama's edge in Cleveland, Columbus and Dayton. Obama needs to trim Romney's advantages in the suburbs and southeastern Ohio's coal country.
The dynamic is similar in Nevada's Washoe County. It's home to Reno, a gambling mecca at the foot of the Sierra Nevadas that is a diverse mix of people, including retired and active-duty military from the nearby Naval air base, Hispanics and transplants from northern California.
If Romney stands a chance of carrying Nevada, he needs to cut into Obama's edge in Las Vegas and run up big margins in the vast ranching and mining country elsewhere. But he stands little chance of beating Obama in Nevada without Washoe County, which accounts for 15 percent to 20 percent of the state's vote.
While Obama has concentrated his Nevada efforts on Las Vegas, Romney has focused aggressively on Washoe County's veterans, and for good reason.
"Washoe is a county where Republicans need to win to have a chance in Nevada," Democratic pollster Paul Maslin said.
But some swing-voting counties don't look like they may be swinging back any time soon.
The best example is Osceola County, a bedroom community to Orlando, Fla., that has experienced an influx in the past decade of Puerto Ricans, Hispanics who are U.S. citizens.
The shift of these typically Democratic-leaning voters, as well as other Spanish-speaking voters in Colorado and Nevada, is part of longer-term trend going in the Democrats' favor and spreading into other neighboring states over the coming generation.
While it bodes well for Democrats in the future, there are no guarantees for Obama this year.
Schneider reported from Florida.
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