WASHINGTON >> If anything is stable in this presidential race, it is the idea that 2012 is a volatile campaign year.

In part that reflects the last two elections, when Democrats and Republicans in turn won big victories, and the expectation that November represents a tiebreaker. Even more, it reflects a roller coaster Republican primary campaign, which at different moments has elevated Michele Bachmann, Rick Perry, Herman Cain, Newt Gingrich, Rick Santorum and Mitt Romney.

So now that Republicans have begun rallying around Romney, does that signal the start of a mercurial, topsy-turvy general election fight with President Barack Obama?

Almost certainly not.

Far more likely, say election experts of all stripes, is a narrowly contested campaign revolving around the mobilization of party loyalists and an attempt to win over a small slice of voters who will remain undecided until almost the end. In other words, get used to numbers like these from the latest New York Times/CBS News poll: 47 percent for Obama, 44 percent for Romney, 9 percent uncommitted.

While either candidate could win, it is “utter nonsense” to expect large gyrations in the voters’ preferences this fall, said Mark Mellman, a Democratic pollster.

Daron Shaw, a political scientist at the University of Texas and a Republican campaign adviser, said “there’s no relationship” between the two phases of the 2012 campaign. Primaries are naturally volatile. They force partisans to choose among candidates of similar philosophies and platforms, which swells the importance of subjective, less predictable factors like personality and messaging.

And early in this Republican primary season, the turbulence was magnified because Romney was matched against a fast-changing cast of alternatives, who dropped out one by one as voters learned more about them.

As the nominating contest progressed from state to state, Santorum, Gingrich and Romney took turns winning various points on the map. Yet exit polls suggested that these candidates did not succeed in shifting opinion, but simply benefited from a state’s demographic makeup. States with more moderate voters tended to favor Romney; states with more evangelical Christian voters tended to vote for Santorum. Gingrich won two contests, his home state, Georgia, and South Carolina, which is next door.

Big swings in opinion are even less likely in the general election. In an era marked by polarization, Mellman noted, most voters are “anchored very firmly” to Republicans or Democrats and grow more so during the campaign.

Obama’s approval ratings have drifted up since last fall, tracking Americans’ growing confidence in economic conditions.

When the Times/CBS News poll last month showed Obama’s approval rating dropping to 41 percent, some analysts saw that as another sign of volatility, this time because of rising gas prices. Obama campaign advisers remain skeptical of the poll’s results, noting that other surveys at the time showed the president’s approval ratings at 50 percent or more.

“The volatility isn’t in the electorate, it’s in the polls,” said Joel Benenson, an Obama campaign pollster. He pointed to both margins of error and methodological differences among the growing roster of publicly available polls.

In significant ways, Obama’s standing evokes memories of the last time an incumbent president sought re-election.

In 2004, George W. Bush’s presidency divided voters along starkly partisan lines. Opinion locked in so early — thanks in part to the proliferation of political media — that John Kerry, the Democratic nominee, did not receive much of the traditional “bump” in the polls after his nominating convention.

As with Bush, Obama’s presidency has divided voters. “He’s locked in much the way Bush was locked in,” said Shaw, who advised Bush’s campaign in 2004. In Times/CBS News polls in the first quarter of 2012, Obama’s share of the vote against Romney has ranged from 45 percent to 48 percent; Romney’s share has ranged from 42 percent to 47 percent.

Shaw doubts it is possible to repeat the swings in opinion that marked the more turbulent races in modern political history. Those include 1968 (Hubert H. Humphrey versus Richard M. Nixon), 1976 (Jimmy Carter versus. Gerald R. Ford) and 1988 (Michael S. Dukakis against the elder George Bush).

That is not to say that the electorate is unmovable. The list of swing states has changed since 2004. Population shifts, including growth in the number of Hispanic voters and moderate suburbanites, have turned generally reliable Republican states like Colorado, Virginia and North Carolina into prime Democratic targets.

Moreover, though the number of “persuadable” voters remains small, it may be slightly larger than eight years ago. Mellman, for instance, pegs that share of the electorate at 5 percent to 10 percent, compared with less than 5 percent when he advised Kerry’s campaign.

Others suspect it could be even more as voters consider the 2008 financial crisis under a Republican president, the deep recession that persisted under the Democratic incumbent and the tentative recovery.

“There’s a lot of churning going on,” said Neil Newhouse, Romney’s campaign pollster. Morris Fiorina, a Stanford University political scientist, envisions a pool of swing voters of 10 percent or more, encompassing working-class social conservatives and more affluent moderates.

Even a small group of undecided voters could significantly change a close, narrowly contested race by breaking sharply enough near Election Day.

Shaw speculated, for instance, that if the economy continues on its upward course, a swing group could break 70 percent to 30 percent for Obama. If swing voters make up 10 percent of the electorate, Obama’s current narrow lead could turn into a November rout of Romney.

An even split of swing voters would keep the race close. But a significant economic reversal could make the undecided group break 70 percent to 30 percent for Romney, wiping out Obama’s lead and leading to a new president in January.