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 Mr. Romney, does that signal the start of a mercurial, topsy-turvy general election fight with President 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 Mr. Obama, 44 percent for Mr. Romney, 9 percent uncommitted.