WASHINGTON -- Over the last 100 years, only five incumbent presidents have lost second-term bids. Mitt Romney is trying to become the 21st century’s first challenger to topple a sitting president, and his camp says it could reach the 270 electoral votes needed for victory this way:

Hold on to the 22 states that John McCain won in 2008. Take back three states that traditionally vote Republican for president but that Barack Obama captured in 2008: Indiana, Virginia and North Carolina. Win traditional swing states Ohio and Florida. Then win one more swing state, such as: New Hampshire, where Romney has a home; Colorado; Pennsylvania; or even Michigan, where his father is remembered as a great governor.

Right now, President Obama and Romney, the presumptive Republican presidential nominee, are virtually deadlocked in national polls. Surveys released Thursday found the candidates in a virtual tie in Ohio and Florida. Polls tend to put Obama slightly ahead in Virginia and North Carolina, and many swing states are well within the president’s reach. Even Arizona – which backed GOP presidential nominee McCain, its senator, in 2008 – could be in play this time; polls there show a virtual tie.

But national polls six months out don’t mean much. The candidates’ campaigns, the national parties and their allied “super” political action committees will saturate the country with advertising wars from now to November. They’ve already begun in select swing states. That could influence public opinion. So might unforeseen events.

The biggest predictor of electoral success is usually a state’s voting history, said Larry Sabato, the director of the University of Virginia Center for Politics. By that measure, he saw North Carolina and Indiana leaning Republican. Virginia, which has seen an influx of urban professionals and immigrants in recent years, now trends Democratic.

Obama won 365 electoral votes in 2008 partly by defying some history. Indiana and Virginia went Democratic for the first time since 1964. North Carolina voted for a Democrat for only the second time since 1964. In those and other states, a combination of energized young and minority voters, as well as white-collar professionals who made up bigger shares of the population, boosted Obama.

Sabato and his staff figure that Obama starts 2012 with 247 electoral votes, “but not all of them are totally secure,” they said in an April 26 analysis. They figure Romney has a base of 206 electoral votes. RealClearPolitics, a nonpartisan website, gives Obama a 253-170 edge – and 270 are needed for victory.

The Romney camp cited a blueprint to get to 270 from Karl Rove, former political strategist for President George W. Bush. It goes like this:

• Win the 22 McCain states, for 180 electoral votes. Romney figures he can hold on to all of them, though Arizona may be questionable. A Rocky Mountain Poll of Arizona voters April 9-17 gave Obama a 42-40 percent lead. He was particularly boosted by Hispanics, who preferred him by 64-25 percent.
• Win Virginia, North Carolina and Indiana, for 39 electoral votes. Indiana appears safely Republican again. To counter Obama in Virginia and North Carolina, Romney needs strong support from conservatives, notably evangelicals. Former Pennsylvania Sen. Rick Santorum won 11 primary and caucus states this year with strong appeal to evangelicals, who still aren’t necessarily convinced that Romney is a worthy candidate. Romney hopes to motivate them May 12 when he gives the graduation speech at Virginia’s Liberty University, an influential Christian institution. “It’s an important stop for conservative political figures,” said Romney adviser Mark DeMoss, a Liberty trustee.