Even two Republicans once on McCain's short list for vice president sounded skeptical.
In a fundraising e-mail on behalf of Senate Minority Leader Mitch McConnell of Kentucky, Mitt Romney referred to "the very real possibility of an Obama presidency."
In the Midwest, Gov. Tim Pawlenty gave a dour assessment of McCain's chances in his state, saying Barack Obama "has a pretty good advantage in Minnesota right now."
Nationally, a poll by the Pew Research Center found Obama with a 16-point lead among registered voters. The survey said Obama had 52 percent and McCain 36 percent, with independent voters supporting the Democrat by a 48-31 margin.
The Nielsen media company reported that both are focusing about three-fourths of their advertising in Florida, Ohio and Pennsylvania. Obama, who had been spending four times as much as McCain on advertising, is now airing only twice as many ads as his rival, the ratings company said.
Those three states are battlegrounds, offering a combined 68 electoral votes on Election Day.
The concentration of firepower comes even as Obama mounts a national advertising campaign that will culminate Wednesday evening with a 30-minute, prime-time commercial on network television. The candidates also planned appearances on cable TV talk shows, including Obama on Comedy Central's "The Daily Show."
The candidates kicked off their final week of campaigning in the southeastern corner of Pennsylvania, which hasn't supported a Republican presidential candidate in 20 years and where Obama is ahead in the polls. McCain is working for an upset and has Pennsylvania as the linchpin to his victory strategy.
"I'm not afraid of the fight, I'm ready for it," McCain told noisy supporters at a rally in this Republican region and home of the world's largest chocolate factory.
Obama's advisers say they are confident of victory in the state. Still, they sent him to rally supporters in Pittsburgh Monday night and to the battleground Philadelphia suburbs on Tuesday. About 9,000 people stood in the mud and a steady, cold rain at Widener University to hear him.
"I just want all of you to know that if we see this kind of dedication on Election Day, there is no way that we're not going to bring change to America," said Obama, uncharacteristically attired in jeans, sneakers and a raincoat. McCain canceled a second event 50 miles away in Quakertown because of the dismal weather.
McCain appeared with running mate Sarah Palin, who planned to stay in the state for rallies in Shippensburg and State College. "Pennsylvania, it's going to be a hard-fought contest here," she said. "It's going to come down to the wire here."
If McCain doesn't win the state's 21 electoral votes, it's hard to see how he can win the presidency since Obama is expected to pick up several of the states that helped re-elect President Bush four years ago. McCain needs one of the blue states to make up for expected losses in the red ones.
Both presidential candidates left Pennsylvania for rallies Tuesday evening in Republican strongholds that have become battlegrounds - McCain to North Carolina and Obama to Virginia.
McCain is increasingly playing defense in states that have been reliably Republican, with the party buying ads in Montana and expanding its advertising in West Virginia.
Early voting in some swing states also appeared to be in Obama's favor. In North Carolina, for example, the turnout for early voting has been nearly a third higher than in 2004 and the number of Democrats has been close to double that of Republicans.
Democratic voters in Florida have numbered about 100,000 more than Republicans, and Democrats hold an edge so far in Colorado.
Florida Gov. Charlie Crist on Tuesday extended early voting hours from eight to 12 hours weekdays, and to a total of 12 hours this weekend, to help ease long lines at polling sites. Early voting there ends Sunday.
McCain told Pennsylvania voters he's the candidate ready to take office, after a military career and years as a prisoner of war. He hammered Obama as a traditional liberal Democrat seeking to redistribute wealth and even willing to displace America's favorite pastime with a 30-minute commercial Wednesday night.
"No one will delay a World Series game with an infomercial when I'm president," McCain said to loud applause.
He said Obama's promise not to raise taxes on those making less than $250,000 can't be trusted after his running mate, Sen. Joe Biden, said in an interview with local television station WNEP that tax relief should only go to "middle-class people - people making under $150,000 a year."
"At this rate, it won't be long before Sen. Obama is right back to his vote that Americans making just $42,000 a year should get a tax increase," McCain said. "We can't let that happen. We won't let that happen."
Obama said a vote for McCain would be a vote for a third Bush term, arguing that their proposals are similar, especially on the economy.
"John McCain has ridden shotgun as George Bush has driven our economy toward a cliff, and now he wants to take the wheel and step on the gas," Obama said.
Obama also cited a comment by McCain's domestic policy adviser, Douglas Holtz-Eakin, regarding McCain's plan to offer consumers tax credits to buy health insurance. Holtz-Eakin told CNNMoney.com that younger voters wouldn't abandon employer-sponsored health care plans because "what they are getting from their employer is way better than what they could get with the credit."
Obama said the comment supported his argument that the individual market is a worse deal. He described the remark as a "stunning bit of straight talk - an October surprisea." Holtz-Eakin said Obama deliberately had taken his comment out of context.
(Copyright 2008 by The Associated Press. All Rights Reserved.)