As Sen. Barack Obama made history last month in Denver by accepting the Democratic nomination for president, his campaign described an ambitious plan to take this year’s contest to states that have traditionally gone Republican for president, expanding the ground upon which the past two elections have been decided.

Then Sen. John McCain seized back the spotlight at the Republican convention in St. Paul, Minn., with the selection of a running mate who played to his reputation as a risk-taking maverick and, in last week’s new round of national polls, helped to return the presidential campaign to a dead heat.

National polls don’t count in the Electoral College, where the next president will be selected. But they do provide useful clues at a given moment to voter attitudes that, roughly speaking, will hold true from state to state. Among independent observers and partisans on both sides, the latest such surveys have produced some agreement on several broad points about the 2008 election:

•Obama and the Democratic Party have some potentially decisive advantages in public attitudes about the issues that will influence votes in November. Top among those is the weakness of the nation’s economy. While the war in Iraq has receded from the headlines, most voters continue to say the invasion was not worthwhile — a dominant issue when Democrats won control of the Congress two years ago.

Closely related is the unpopularity of a Republican president at the close of his second term. Obama’s campaign has therefore worked hard to portray McCain as promising “more of the same” — a third term of Bush policies that Democrats view as self-evident failures.

•Obama has made great strides in moving from the status of virtual unknown to a position of historic significance in American politics. But because he does not have a long record of experience as an elected leader, Obama has not finished the job of convincing a solid majority of voters that he should be the nation’s chief executive.

•McCain’s military background and his long Senate record in foreign affairs has helped him to retain the Republican Party’s key traditional advantage on national defense. For the moment at least, McCain appears to have solidified his standing as a prospective wartime commander-in-chief by stressing this record — from service in Vietnam to support for a strategic shift (the “surge”) that has helped to bring a marked improvement in the military situation in Iraq.

Among conservatives who had harbored doubts about him, McCain’s startling choice of Gov. Sarah Palin of Alaska has burnished his image as a bold leader. What remains to be seen is whether Palin’s presence on the GOP ticket will yield durable gains among voter groups — white women, for example, and blue-collar workers — that Obama cannot afford to lose by significant margins.

•For all these reasons, the race is very close today — in nationwide voter polls, in snapshots of the electorate vote matchups, and in many statewide polls of the so-called “battlegrounds” that could swing either way.

“I’m not sure I’d bet against Obama,” said Charlie Cook, an independent campaign analyst based in Washington. But, he said, the conventions have broken a distinct pattern set after Obama sewed up the Democratic nomination last spring. “Obama was never way ahead, but boy was that lead steady,” Cook said of polls that had the Illinois senator up by a few points over McCain through much of the summer.

Since the conventions, he said, that trend has shifted — perhaps temporarily and certainly not decisively. “But you have to wonder if the underlying chemistry has changed,” Cook said of the post-convention polls that show improvement for McCain: “We have to wait and see if they have lasting meaning.”

The Web site Real Clear Politics, which relies on averages of leading polls, made the race for electoral votes Obama 217, McCain 216, as of Friday, with 105 votes up for grabs among the 270 needed to win.

Cook’s latest analysis (published before the latest polls showed McCain’s post-convention gains) gave Obama an advantage of 240 electoral votes to 185. Cook put 113 electoral votes in the toss-up category.

A look at the map suggests that Obama’s bid to stretch this battleground for electoral votes is built on a solid foundation. More voters identify themselves today as Democrats than as Republicans, a basic shift from the rough parity of four years ago. Obama has therefore poured money and organizational resources into the Republican “red” states of previous elections that might turn Democratic blue this year.

Besides such familiar battlegrounds as Ohio, Pennsylvania, Florida and Iowa, Obama campaign manager David Plouffe last month mapped out the case for contesting such states as Virgnia, North Carolina, Nevada, Colorado and New Mexico. Even Montana and North Dakota were on the list; Plouffe argued that it made sense to invest in all these potentially winnable states even though McCain would probable carry some of them in the end.

Cook agreed: “What we’re talking about is states that Obama doesn’t have to win. But if McCain loses, that is bad for him.” Thus, he said, “Colorado was mentioned in passing four years ago but it’s up for grabs this year. Virginia was glanced at years ago. Now it’s a major battleground. ‘’

Independent analyst Ed Sarpolis said Florida “will be tough for Obama,” which is why New York Sen. Hillary Rodham Clinton will appear there often to appeal for the votes of displaced Northerners, including the state’s large bloc of Jewish voters.

Michigan and Virginia show some of the crosscurrents affecting many battleground states.

In addition to expanding the turnout of black voters, Obama’s plan is to consolidate the gains that the Democratic Party has made in recent years in Northern Virginia. But as McCain demonstrated when he and Palin drew the biggest Republican rally of the year to the suburban Washington city of Fairfax, his background still counts for a lot in Virginia, which has 13 electoral votes.

“This is a veteran-oriented state, a military-oriented state,” said political science Prof. Larry Sabato, of the University of Virginia. “If Obama wins here, it would be an upset.” Nevertheless, Sabato said, there is no doubt about the state’s gradually shifting partisan cast in favor of the Democrats. Michigan, with 17 electoral votes, is one of the few states that McCain currently can count as a potentially telling switch in his favor. But there, he is an underdog, according to Sarpolus, an independent analyst based in Michigan.

Sarpolis said a close look at the state shows how big a challenge McCain faces in the state — and why his recent gains can be crucial if they represent durable trends.

“The Democrats can’t win the White House without Michigan,” he said.

In 2000 and 2004, President Bush led in Michigan but ended up losing in that state because he carried under 10 percent of the black vote and only 52 percent of the white vote. Assuming that Obama will be at least as strong among black voters as his Democratic predecessors, McCain will need at least a 55 or 56 percent share of the white vote “to offset Obama’s advantage among minority groups,” Sarpolus said.

Past elections are as instructive on the role of voting issues in Michigan as they are on voting bloc trends, Sarpolis said. “Kerry turned it around when he stopped playing by Bush’s playing book— on terrorism and war, safety and security — and started to focus on health care,” he said. Kerry thus drew a critical share of Michigan’s older voters and carried the day.

For now, Cook is among the analysts who theorize that the Republican convention worked better for McCain than the Democratic convention did for Obama.

“The Democratic convention was beautifully produced and ran like clockwork,” he said. “But it’s beginning to look like a really good Chinese meal. It looked good, it tasted good, it felt good at the time,” Cook said. “But, gee, I’m still hungry. It didn’t sustain. I’m not sure we knew anymore of what we needed to know about Obama on Friday morning” after his speech, “than we did before the thing started on Monday.”

But practically from the moment that Palin took the stage in St. Paul, “She has been like a B-12 shot for McCain,” Cook said. Before the Republican convention, the shorthand for Obama against McCain was “Change against the Status Quo,” Cook said. Now in the minds of many voters, “it’s Change against Change.”

The upshot, Sarpolus said, is this: “It’s going to come down to a very close race — like it did with Kerry four years ago and Gore eight years ago.”

jmulligan@belo-dc.com