WASHINGTON – Stubbornly close and deeply divisive, the presidential race throttles into its last 100 days as an enormous clash over economic vision, with the outcome likely to come down to fall debates, final unemployment numbers and fierce efforts to mobilize voters.

It may seem like an election for the whole nation, but only about eight states will decide who wins the White House.

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Polling shows the contest between President Obama and Republican Mitt Romney remains remarkably static across the country and in those pivotal states, even as both men and their allies pour money into largely negative television advertising to sway opinions.

The most contested states are Colorado, Florida, Iowa, Ohio, Nevada, New Hampshire, North Carolina and Virginia. Pennsylvania is also in the mix.

The state of the race again shows how certain states take on outsized importance in a contest that is decided by electoral votes, not the popular vote.

The upcoming stretch is loaded with opportunities for the candidates to capture the public’s imagination, land a big blow or flub a chance. Romney is closing in on his vice presidential nominee, both candidates will give highly scrutinized convention speeches, and the two will face off three times in October debates.

Then there are the surprises that can jolt the campaigns and test the candidates.

“We’re all looking for that moment,” said David Gergen, a political analyst who has advised Republican and Democratic presidents.

He predicted it could come in the first of the debates, in Denver on Oct. 3, when Obama and Romney finally stand on a stage together and go at it over economic policy.

Gergen said it could be the most defining debate in more than 50 years.

“Obama is leading, but it’s often 47-45. He’s still got to get to 50,” he said. “If the undecided voters all break at the last minute, that could go against the incumbent. If Obama wants to wrap it up, the first debate carries enormous significance.”

The heart of the campaign is still an important choice of economic visions. The tone and the substance often have been far more narrow and biting.

“Sometimes politics can seem very small,” Obama says in one of TV ads, reaching for the positive. “But the choice you face couldn’t be bigger.”