CHICAGO (Reuters) - Barack Obama leads John McCain in five of eight crucial battleground states one week before the presidential election, with McCain ahead in two states and Florida dead even, according to a series of Reuters/Zogby polls released on Monday.

Obama held steady with a 5-point lead over McCain among likely U.S. voters in a separate Reuters/C-SPAN/Zogby national tracking poll, the same advantage he held on Sunday. The national telephone poll has a margin of error of 2.9 percentage points.

Republican McCain is struggling to defend about a dozen states won by President George W. Bush in 2004, including all eight of the states surveyed over the last three days.

Breakthroughs by Obama in any of those states could move him close to or above the 270 electoral votes he needs to win the White House on November 4.

Obama, a Democratic senator from Illinois, held narrow leads over McCain in Virginia, North Carolina, Missouri, Ohio and Nevada, most within the margin of error of 4.1 percentage points. McCain had a solid 10-point lead in West Virginia and a 6-point edge in Indiana.

The two candidates were tied at 47 percent in Florida, the largest of the battlegrounds with 27 electoral votes and the state that decided the disputed 2000 election.

Most polls show Obama comfortably ahead in all of the states won by Democrat John Kerry in 2004, but the Reuters/Zogby polls show McCain in serious danger in several states won by Bush.

“If Obama holds the Kerry states, he is in line now to get enough electoral votes to win the White House,” Zogby said, noting McCain faces a difficult fight in a handful of states where Republicans have a long history of success.

‘UPHILL BATTLE’

“These polls are a measure of what an uphill battle McCain faces to win,” Zogby said. “These are all Republican states and McCain has a very tough challenge, but they are all close.”

The state polls showed Obama leads in Virginia by 7 points, 52 percent to 45 percent, and in neighboring North Carolina by 4 points, 50 percent to 46 percent. In Missouri, Obama leads by a narrow 48 percent to 46 percent.

In fast-growing Nevada, an influx of new residents, growth in the Hispanic population and economic troubles have given Obama momentum and a 4-point lead, 48 percent to 44 percent.

Obama leads by 5 points, 50 percent to 45 percent, in Ohio, the state where Bush’s narrow victory over Kerry clinched his re-election in 2004.

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But Obama’s hopes of putting heavily Republican Indiana into the Democratic column are threatened as he trails 50 to 44 percent. McCain has a comfortable 50 percent to 40 percent lead in West Virginia, a state some polls had shown was becoming competitive.

With the race in Florida tight, both candidates plan to spend several campaign days there down the stretch. Obama will hold a late-night rally in Orlando on Wednesday with former President Bill Clinton.

Obama has built his edge in the battleground states much as he has in national polls. In the latest national tracking poll, he leads McCain by 13 points among independents and by 12 points among women.

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McCain leads handily in the national poll among whites by 54 percent to 41 percent, but Obama has done a better job of reaching across the partisan divide -- he attracts 19 percent of conservatives while McCain wins just 5 percent of liberals.

Independent Ralph Nader and Libertarian Bob Barr both received support from 1 percent of those polled nationally. Two percent said they remain undecided in the race.

The rolling tracking poll, taken Friday through Sunday, surveyed 1,203 likely voters in the presidential election. In a tracking poll, the most recent day’s results are added, while the oldest day’s results are dropped to monitor changing momentum.

The U.S. president is determined by who wins the Electoral College, which has 538 members apportioned by population in each state and the District of Columbia. Electoral votes are allotted on a winner-take-all basis in all but two states, which divide them by congressional district.

The state polls were taken Thursday through Sunday and surveyed 600 to 603 likely voters in each state, with a margin of error of 4.1 percent.