And the stakes are enormous for both of them, if only because more Americans will tune in than at any other time so far in the campaign, perhaps 80 million in all.
"This could really shape up to be quite a challenge," said Paul Begala, the Texas political adviser who has been helping Mr. Gore prepare.
The two rivals, who barely know each other, have been studying the opponent from afar for months in a race that is dead even in most of the public opinion polls.
At 8 p.m. Tuesday, they'll share the same stage for the first time in Boston and, over 90 minutes, size each other up and make their cases to be president.
Mr. Gore and his aides say he wants to use the forum to speak more to the American people than to Mr. Bush, but nonetheless draw the differences on the issues.
Mr. Bush also wants to emphasize his differences with Mr. Gore, the governor's aides said, but he is intent on raising the character issues of trust and integrity, as well.
The vice president is "relentless in attacking," Mr. Bush's communications director, Karen Hughes, told reporters aboard the Bush campaign plane Monday. "We expect scripted, Hollywood-style debate lines."
Mr. Gore, though, has likened the debate to a job interview in which he is trying to get hired by the American people. And the Republican governor, too, sees it as an enormous opportunity â€“ to "speak directly to the people" and explain his vision for America.
Both have vowed an issues-oriented forum, with each staking out his positions on education, defense, the economy, taxes, Medicare, prescription drugs and Social Security.
But there will be other factors at work, as well.
Analysts suggest that viewers will be watching to see if Mr. Bush, midway through his second term as governor, has the experience and gravitas to be president. And they'll be watching to see if Mr. Gore, a sometimes-distant candidate, can connect with the voters.
Mr. Gore's oldest daughter, Karenna Gore Schiff, said Monday that her father was buoyed by the suggestions from a dozen Middle Americans he had met on the campaign trail and invited to Sarasota, Fla., to help him prepare for the debate.
One had urged him just to be himself, she said on NBC's Today show. Another urged him to spell out his programs in detail. And still another wanted him to make sure his tie was straight.
"People have a range of opinions," Ms. Schiff said. "But it's really fun for my dad to get that feedback."
Appearing on the same show, Mr. Bush's campaign chairman, Don Evans, said Tuesday night's debate would be particularly important because many voters would be paying attention for the first time.
"America will see a man that is a visionary, a man that understands government," Mr. Evans said. "They'll see a man who is prepared to lead this great country."
The vice president, who has been preparing since Saturday at the Mote Marine Laboratory in Sarasota, went through his final pre-debate drills Monday. Plans for a quick campaign event were scrubbed, and he appeared in public for photographers only briefly during a morning walk on the beach.
Mr. Bush, who prepared at his Central Texas ranch over the weekend, headed for Boston on Monday, with an overnight campaign stop in Huntington, W.Va.
At a rally on a coal barge moored to the banks of the Ohio River, Mr. Bush urged his supporters to help send him to the White House.
"We're five weeks away from changing Washington, D.C.," he said. "We're five weeks away from a new attitude in Washington."
Joined by members of the United Mine Workers union wearing hard hats, Mr. Bush said he supported "working people" in the coal industry while Clinton-Gore administration energy policies have benefited "big foreign oil."
Anticipating a likely line of attack from Mr. Gore in Tuesday's debate, Mr. Bush defended his plans to use the budget surplus to bolster Social Security and provide a 10-year, $1.3 trillion tax cut.
"Tomorrow night in the debate, he'll say, 'Oh, you can't do that,'" Mr. Bush said. "You know why? Because he trusts government, and I trust the people."
Tuesday's debate is the first of three presidential forums. Mr. Bush and Mr. Gore are scheduled to meet again Oct. 11 in Winston-Salem, N.C., and Oct. 17 in St. Louis. The vice presidential nominees, Republican Dick Cheney and Democrat Joe Lieberman, debate Thursday night in Danville, Ky.
For Mr. Bush and Mr. Gore, the first debate could be compared with their acceptance speeches at this summer's national conventions â€“ another potentially pivotal event â€“ but with three times the audience.
On this new stage, performance will count, perceptions will matter more and a misstep could be disastrous.
Sometimes, though, "people will see what they want to see ... see things that corroborate what they feel," political analyst Charles Cook said. And he suggested that as Mr. Gore rebounded in the polls after the Democratic convention, some questions about Mr. Bush's capacity to be president may have "hardened."
"The bar that he has to cross is higher today than it was a month ago when people really liked George Bush and they really didn't care for Al Gore at all," Mr. Cook said. "He only had to get a minimum threshold of competence, of perceived competence, to get elected. And now that people see them relatively equally in term of favorability, then I think that lifts the bar."
On the other hand, analysts noted that Mr. Bush and his allies have played the expectations game well, relentlessly portraying Mr. Gore as an experienced, tough debater â€“ so that if Mr. Bush just survives, he could claim victory.
"The expectations are that Gore's going to mop the floor with him," said political scientist Robert Johnstone, a presidential scholar at Earlham College in Indiana.
"But the danger for Gore is that he gets so wrapped up in a sort of prosecutorial mode that people think of him as a prosecutor rather than a candidate. That'll turn people off."
Generally, Mr. Bush has benefited from low expectations because "it's easy for him to exceed them," said Don Sipple, a Republican political consultant who advised Mr. Bush in his 1994 gubernatorial race against Ann Richards.
"Clearly in the last 10 days, he's got his legs back," said Mr. Sipple. "Bush operates on confidence. When he is confident, he's strong. And when he's not, he's shaky. The enduring trait is, what you see is what you get."
Over the weekend, Mr. Gore teased reporters by saying he was planning a "completely different approach" to Tuesday night's debate, without offering any details.
His aides say that he may single out several of his special debate advisers, who will follow him to Boston on Tuesday, in an effort to connect some of his proposals with "real people."
If nothing else, analysts say a must-meet goal for each candidate is to avoid the kind of mistake that could become a defining sound bite on television.
"The undecided people will see the highlights, and if there are highlights â€“ or more accurately lowlights â€“ they will have an impact," said Chuck McDonald, a Democratic political consultant in Texas.