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.