Chocorua, N.H. ? And you thought the presidential politics story in New Hampshire ended six months ago.

But that was only the frontispiece, an evocative landscape like the famous one — silvery lake, soaring mountain, still one of the most stunning roadside attractions in this state — that the artist Thomas Cole painted here in 1827. It turns out there will be far more to the Granite State political story than even Mitt Romney, winner of the Republican primary in January, could have contemplated.

These days Romney, who served as governor in the neighboring state of Massachusetts and has a lakeside vacation house in Wolfeboro, a pleasant 40 minute drive from here, is running hard in New Hampshire again. So is Barack Obama, who took the state four Novembers ago.

Together the president, who was in Durham last week, and Romney, who was in Stratham only a few days earlier, are conducting an unlikely but vital battle for a mere four electoral votes. Both sides believe the November election will be that close.

A generation ago, no one would have thought that New Hampshire, with its sturdy Republican tradition, could possibly be a presidential battleground once the candidates, like characters in “Brigadoon,” a summer-stock favorite here for nearly a half century, packed up their dial telephones, index cards and metal buttons and moved on to the next stage. Between Franklin Roosevelt’s last campaign in 1944 and Bill Clinton’s first in 1992, New Hampshire voted Republican every time but in the 1964 Lyndon Johnson landslide. Even then this county went for Barry Goldwater, the only one in New England to do so.

But since then this state, once resolutely red, has turned purple, which by a sort of perverse poetic justice is the very color of the sky in Cole’s Chocorua oil landscape. Clinton won the state in 1992 by a hair, and then Gov. George W. Bush seized it by just as slim a margin — but would have lost both the state, and the 2000 election, had not Ralph Nader taken about 22,000 votes, almost all of them from Vice President Al Gore.

John Harrigan, a veteran North Country newspaperman, has described New Hampshire as “a jumbled geography of mountains, valleys and ridges, more than 90 percent woods and water, peopled by relatively few individuals, mostly unposted and open to all.” It is the openness that defines the place, even though its people are famously closed — to outside fashions and frippery.

Now it is open to changing colors, from red to blue and then back again twice, and the irony is that this year’s election is between two men who were defeated in primary fights here in 2008 and left for dead, only to recover, Obama later that year and Romney in four years’ time.

The velocity of the change in staid old New Hampshire has been stunning, which is why Romney’s forces believe they will prevail here — a notion that has prompted Obama to intensify his organizational efforts.

Two years ago, Democrats controlled the state House (224-176) and the state Senate (14-10), only to become the victims of a stunning GOP surge that gave the Republicans overpowering margins in both, 293-104 in the House and 19-5 in the Senate. Meanwhile, the Republicans took back two congressional seats, elected a senator to an open seat and overturned a 3-2 disadvantage on the Executive Council, an institution with colonial antecedents and functions so peculiar and inscrutable that no other state has copied it, and now have a 5-0 margin there.

“This tells us,” says former Gov. John H. Sununu, White House chief of staff under the elder President Bush, “that if you run good candidates and put in the requisite effort on the ground, this can be a Republican state.”

Not so, says Raymond Buckley, an 18-year veteran of the state House who is chairman of the Democratic Party here. “That’s a lot of locker-room talk,” he says. “They know they’re going to lose and are trying to pump themselves up for the big game. It’s very eighth grade of them, and it has really hurt the Republican brand.”

Republicans are planning to emphasize their nominee’s familiarity with the state and his opposition to taxes, part of the state’s unshakable orthodoxy since almost the beginning of time. Senior Republicans believe Romney’s profile has a happy resemblance to that of Judd Gregg, a former governor and senator who as a fiscal conservative had great credibility in Concord and Washington, a flinty but effective advocate for budget restraint and low taxes.

Republicans have won 29 of the last 39 presidential elections here, but the Democrats are preparing a withering critique of Romney’s gubernatorial years and will attack what they describe as Republican obstructionism in the capital.

Ordinarily this is a pretty state in the autumn, all bright colors and crisp air and the bracing coolness of fresh apple cider at every farm stand. But this fall will not be pretty.

That’s because both parties conduct significant primaries here, so the level of political organization is more developed than most anyplace else.

Indeed, politics has a different flavor here, befitting a state where it is possible, at Sherman’s Farm Stand in East Conway, to buy a quart of blueberry milk, which tastes better than it sounds.

The independent vote is important in New Hampshire primaries. Non-aligned voters can bounce from one party’s ballot to the other’s, assuring that both parties reach across the political spectrum in a way that they don’t elsewhere. In a general election, independents can provide a third or more of the votes. And the farther you get from the primary, the more the independent vote rises, because newly enrolled voters tend to be undeclared. These independents are a highly motivated and highly courted part of the electorate.

So while the conventional wisdom renders the bigger states of North Carolina, Virginia, Colorado and Ohio as the big prizes in November, smart eyes also are on this state. As New Hampshire goes, so goes the nation — or so the theory goes in early summer. Pass the blueberry milk and prepare for battle.