"In Washington, sometimes when times get tough, people get a little nervous," Mr. Bush told reporters, who quizzed him over new surveys showing that his once-dominant lead has evaporated.
"Listen, Al Gore is a very tough opponent," Mr. Bush said. "He is the incumbent. He represents the incumbency. And a challenger is somebody who generally comes from the pack and wins, if you're going to win. And that's where I'm coming from."
Seeking to regain his footing, Mr. Bush called out some big guns â€“ former retired Army Gens. Colin Powell and Norman Schwarzkopf â€“ to join him in trying to move the day's theme to rebuilding the military, an issue that traditionally favors Republicans.
Mr. Bush shrugged off a mounting critique of his campaign efforts by some leading Republicans, who worry that the Texas governor has lost the initiative on policy issues, failed to capitalize on his selection of running mate Dick Cheney and allowed a dispute over presidential debates to become a distraction.
Some Republicans in Congress are "a little fidgety," said House Republican Conference Chairman J.C. Watts of Oklahoma. While acknowledging that Mr. Bush's team has been on the defensive recently, Mr. Watts said he remains well-positioned to win.
"I'm not jumping off a bridge because he's had a tough two weeks," he said. "I have no question that they are going to come back."
Said Rep. John Kasich of Ohio, a former GOP presidential hopeful: "It's just a matter of getting [the message] articulated in a more clear way, in a strong way."
Mr. Bush, recalling his 1994 gubernatorial run against incumbent Ann Richards in Texas, noted that then also, he trailed a formidable foe in the polls around Labor Day, but he went on to victory.
"I've been down in the polls before in this campaign, and I've been up," he said. "And every time I went up, a lot of the folks in Washington were right with me. And when the polls went down, they started getting a little nervous."
But Mr. Bush said he is just "fixing to get into the heat of the contest" in which he declared: "I am the underdog â€“ I sure am. But I was the underdog when I first started. Nothing's changed."
Still, Mr. Bush said he plans to retool his campaign, including town hall meetings and gatherings at coffee shops or in the homes of voters, to better humanize his tax-cut message and emphasize "my ability to relate to people."
Spokeswoman Karen Hughes said the campaign is planning to unveil the slogan: "Real plans for real people."
Also Thursday, Mr. Bush softened his all-or-nothing stand on presidential debates, conceding that Mr. Gore likely would not accept his terms.
"Evidently, he's not going" next Tuesday to NBC in Washington, where Mr. Bush wanted their first meeting. "But there's going to be debates, and we'll work it out," he said. "You cannot run for the presidency without presidential debates."
Mr. Bush had initially refused to take part in three 90-minute forums sponsored by a bipartisan debate commission, offering instead to meet Mr. Gore in one of those debates and in two other forums on NBC's Meet the Press and CNN's Larry King Live.
But on Thursday, he indicated that he is willing to negotiate.
Republican Rep. Ray LaHood, a Bush supporter from Illinois, said the GOP nominee has been slowed by such wrangling, calling "the debate thing a distraction."
Meanwhile, several polls Thursday had Mr. Bush, who held a double-digit lead earlier in the summer, now either tied with or trailing Mr. Gore.
Mr. Gore led 47 percent to 44 percent among likely voters in a CNN-USA Today-Gallup poll, which had a margin of error of 4 percentage points. The candidates were tied at 47 percent in an ABC-Washington Post poll of likely voters, with an error margin of 3.5 percentage points.
A new survey by independent pollster John Zogby had Mr. Gore leading Mr. Bush 45.6 percent to 40.2 percent, with a margin of error of 3.3 percentage points.
And a new Texas Poll shows that although Mr. Bush has a commanding lead in his home state, it has eroded over the summer. The survey shows Mr. Bush leading Mr. Gore, 53 percent to 30 percent. That is down from the spring, when he was ahead 57 percent to 24 percent. The poll, conducted Aug. 7-30, has a 3 percentage-point margin of error.
"I guess some of my supporters wanted it to be a runaway," Mr. Bush told reporters as he prepared to leave Michigan. "The vice president is running a strong race, but so am I. I'm under no illusions, and neither should our supporters be."
Charles Black, a longtime GOP consultant and a Bush adviser, said Republicans "got cocky or complacent" after their convention. Now, he said, Bush backers "just need to bear down and go do their homework."
GOP operative John Weaver, a chief consultant for Bush primary rival John McCain, said there is reason for concern after watching Mr. Bush get knocked off course.
"What the Bush campaign can't afford to let happen is for this to kind of freeze in before the Olympics and then come into October facing a 7- or 8-point deficit," he said.
The latest criticisms are similar to those Mr. Bush faced in the spring when some Republicans, upset at a series of stumbles, questioned whether his Austin-based political team could run a national campaign.
"That's Washington," Mr. Bush responded Thursday. "That's the place where you find people getting ready to jump out of the foxhole before the first shell is fired. But when you get out here in Michigan or Ohio or Pennsylvania and you talk to the troops on the ground, they're excited and they want to win."
For the second day, Mr. Bush on Thursday said the Clinton-Gore administration has let the nation's defense slip.
Flanked by Mr. Powell at a VFW hall in Westland, Mr. Bush said "there are troubling signs" in the readiness of America's military. "Our troops aren't ready. ... And leadership does not ignore troubling signs."
In Dayton, Ohio, Mr. Schwarzkopf joined Mr. Powell and Mr. Bush on stage for a rally on a college campus.
Although Mr. Bush did not say that Mr. Powell would be part of his administration, he clearly left the impression. "If all goes well," he said, nodding toward the retired general and winning applause from the veterans.
Mr. Cheney, a former defense secretary under President George Bush, underscored the national security pitch at two defense plants in New England.
In Maine, Mr. Cheney blamed the Clinton administration for low morale and shrinking budgets. He said that the day after the Persian Gulf War ended, he called former President Ronald Reagan to thank him for investing in the military a decade earlier.
"Is anybody going to call Al Gore and Bill Clinton in the future and thank them for what they've done? I don't think so," he said.
The Gore campaign said the vice president proposes spending $100 billion of the projected federal surplus on the military over 10 years while Mr. Bush plans on spending $45 billion.
Staff writers Todd J. Gillman, Sam Attlesey and Michelle Mittelstadt contributed to this report.