With the November field set, Americans can look forward to months of trench warfare as President Obama and Republican rival Mitt Romney wage a costly, brutal and scathingly negative fight over a narrow slice of voters in a limited number of states.

Although Romney still must accumulate the 1,144 delegates he needs to officially claim the Republican presidential nomination — a task made infinitely easier by the exit of his chief rival, Rick Santorum — the more important calculation involves the 270 electoral votes needed to capture the White House.

Each candidate starts with a core of partisan supporters, which together represent at least 80% of the electorate, and a set of states he can reliably expect to win come November, strategists in both camps agree.

Romney is almost certain to sweep most of the Great Plains and Southern states, notwithstanding his weak performance there during the primary season, as contempt for Obama overcomes much of the resistance the former Massachusetts governor has faced among evangelical and socially conservative voters.

The president should repeat his 2008 performance by prevailing along the West Coast and carrying New York, his home state of Illinois and most of New England and the Mid-Atlantic states.

That leaves about a dozen states to decide an election that, barring the unexpected, promises to be more like the cliffhangers in 2000 and 2004 than Obama’s comfortable victory four years ago.

Burdened by stubbornly high unemployment and a touch-and-go recovery after the Great Recession, the president faces a stiff challenge holding onto many of the states he carried in 2008, starting with Florida and Ohio.

Both are perennial targets: Florida served as tiebreaker in the fiercely disputed 2000 presidential race, and no Republican has won the White House without carrying Ohio. That is one reason the state’s freshman U.S. senator, Rob Portman, is believed to be on Romney’s shortlist of potential vice presidential running mates.

Obama is also battling to keep toeholds in the South and interior West, two areas where he outperformed recent Democrats, carrying North Carolina, Virginia, Colorado and Nevada. His narrow win in North Carolina was the first for a Democrat in 32 years. Strategists hope to boost Obama’s prospects by using this summer’s nominating convention in Charlotte to rally support.

Nevada is another cause for White House concern, given its punishing jobless and home foreclosure rates. Romney handily won the Republican caucuses in February and can count again in November on strong support from the state’s large Mormon population.

Other tossup states look more promising for Obama. Pennsylvania and Wisconsin have gone Democratic in the last several presidential elections, although not without a fight.

That leaves three genuine swing states — New Mexico, Iowa and New Hampshire — which each party has carried at least once in the last three presidential campaigns. Of those, Romney may have the best shot in New Hampshire, where he owns a home and is well-regarded from his years as governor next door.

Even with a limited map, both candidates say the election will be — or should be — about big things. (Last week, however, the flap over stay-at-home moms illustrated the inevitable digression into matters that have little to do with voters’ main concern, which remains the economy.)

Addressing newspaper editors this month in Washington, the two previewed their strategies and messages for the general election.

Obama, appearing a day ahead of Romney, spoke of economic fairness and condemned “trickle-down” policies that benefited the few and fortunate and, in his view, caused the joblessness and other hardships he inherited as president.

“Can we succeed as a country where a shrinking number of people do exceedingly well while a growing number struggle to get by?” Obama asked. “Or are we better off when everyone gets a fair shot?”

He tied Romney to the unpopular Republicans in Congress, citing Romney’s embrace of the House GOP’s budget proposal. Obama called that proposal “thinly veiled social Darwinism” that would gut spending on programs like education, government research and job training.

Romney responded by blaming Obama for the tepidness of the economic recovery and said he would spur growth by cutting taxes across the board, repealing regulations he deemed onerous and boosting domestic energy production “so that we finally get the energy we need at a price we can afford.”

He defended the GOP budget plan, calling Obama’s statements hyperbole, and accused the president of shrinking from the fiscal challenges posed by the rising costs of Medicare and Social Security. Romney vowed to reform both, without harming today’s seniors.

“Instead of growing the federal government, I will shrink it,” Romney said. “I will repeal ‘Obamacare’ ” — the sweeping healthcare bill the president signed into law — “and cut programs that we cannot afford.”

Neither candidate, however, is operating from a position of political strength.

Obama’s approval ratings have been mediocre and voters continue to express unhappiness with his handling of the economy, making him highly vulnerable to a strong challenge. Romney, however, has proved a weaker candidate than many expected, bumbling his way through the primary season and ending up less popular than when he started, especially among women and independent voters.

For each candidate, it may be easier to assail his opponent than make an affirmative case on his own behalf, and with hundreds of millions of dollars to spend, there will be plenty of nastiness to go around.

“Super PACs,” the nominally independent fundraising groups that support candidates, have already gotten started; last week, the better-funded Republicans began airing ads against Obama in several key states.

Paradoxically, the voters both sides are pursuing — those not reflexively backing or altogether sold on either candidate — are often the ones turned off by name-calling and negativity and could end up staying home. That, in turn, would boost the influence of partisans on both sides.

While some try to affix clever labels — soccer moms, NASCAR dads — they are men and women of all ages, incomes and education levels and generally have one thing in common.

“They want solutions,” said David Winston, a Republican pollster. “They’re tired of people blaming the other side. They say, ‘No. Just tell us what you’re going to do.’ ”

mark.barabak@latimes.com