RELATED: Obama-Romney will drive Virginia Senate race

President Obama officially kicked off his re-election campaign Saturday in Richmond, marking the fourth straight day of presidential politicking in Virginia, the newest must-win battleground state on the electoral map.

It was Obama's second trip to the Old Dominion in as many days and followed two days of campaigning in Northern Virginia and Hampton Roads by his likely Republican opponent, Mitt Romney.

"This may well be the state that decides who the next president is," Romney said at a campaign stop in Portsmouth with Gov. Bob McDonnell at his side. "You're going to hear it all, right here in Virginia."

For nearly half a century, Virginia was a Republican stronghold, voting in line with its Southern neighbors in every presidential race since 1964. That changed in 2008, when Obama beat Republican Sen. John McCain by 6 percentage points -- a margin more decisive than his victories in Ohio and Florida, the battlegrounds so pivotal in recent presidential contests.

Virginia's transformation was decades in the making, fueled by a population boom in the Northern Virginia suburbs of Washington that brought with it cultural and ethnic diversity that realigned the state's political posture, political experts and campaign consultants agreed.

"Some states are on a pendulum that swings back and forth between Republican and Democrat," said Mo Elleithee, a long-time Democratic strategist. "I think in Virginia, the pendulum stopped right in the middle."

But Obama himself has helped make Virginia more competitive, too.

He won about a dozen counties in 2008 that Democrat John Kerry lost to then-President George W. Bush four years earlier. Obama's historic candidacy boosted turnout in Democratic strongholds like Fairfax County and Richmond. And traditionally disinterested young voters turned out in droves to help elect the nation's first black president, lifting Obama to victory in Republican areas with college towns, places like Blacksburg, the home of Virginia Tech.

In Washington's outer suburbs of Loudoun and Prince William counties, Obama appealed to burgeoning minority communities and out-of-state transplants to win areas that overwhelmingly favored Republicans in nonincumbent elections like 1992 and 2000. Those counties could decide which way Virginia goes and are must-wins again for Obama.

"They're symbolic for the state as a whole. If Obama is winning those two counties, he probably will win the state again," said Geoff Skelley, of the University of Virginia Center for Politics. "I do really feel Virginia is Obama's firewall, and if he loses Virginia, it's more likely than not he'll lose the election."

Obama's campaign already has 13 field offices in Virginia, many of them in areas won by both Obama and Bush. Romney will open an office in Arlington as soon as next week, said Pete Snyder, chairman of Virginia's Victory 2012 Campaign.

"The population demographics have changed," Snyder said, "but it's winnable for a conservative candidate that focuses like a laser on the economy."