Tomorrow in Western Pennsylvania, Sen. Barack Obama will kick off his campaign as the Democratic Party's presidential candidate.

On Saturday, Sen. John McCain - perhaps with his newly named running mate - will touch down in the state for the 16th time since becoming the Republican Party's presumptive nominee.

And that is just the beginning. Both parties agree there will be a blizzard of presidential campaign stops in Pennsylvania between now and November. Once again, the state is a crucial election battleground; in June and July, for instance, more TV ad money was spent here than in any other state.

The consensus among political leaders and pundits is that the state will play a decisive role in determining who wins the White House - with Obama having more at stake. If the Illinois senator loses the state, with its 21 electoral votes, they say, he loses the election.

"The Keystone State is key," said Larry Sabato, director of the University of Virginia Center for Politics. "Obama can't win without it."

Montana Gov. Brian Schweitzer put it even more bluntly when speaking earlier this week to members of the Pennsylvania delegation at the Democratic National Convention in Denver: "We're counting on you, Pennsylvania. If you don't deliver your votes to Barack Obama, it's over before the polls even close in Montana."

The Obama campaign is making no secret of how important a role Pennsylvania will play in its overall political strategy. And Obama is making that clear by picking Beaver in Western Pennsylvania as the launching ground for his general-election campaign.

At tomorrow night's event, where Obama will meet with voters, he will be joined by his wife, Michelle Obama. With them will be vice-presidential candidate Joseph R. Biden Jr. and his wife, Jill Biden.

The entourage then will board a bus for weekend appearances in two other battleground states, Ohio and Michigan.

Sean Smith, Obama's spokesman in Pennsylvania, said: "The fact that the first stop we're making is in Beaver shows how seriously the Obama campaign is taking Pennsylvania.

"We know this race is going to be close, and we know Sen. McCain will be making an all-out effort to win the state," Smith said. "But we like our chances here."

Political analysts believe Obama has some clear advantages in Pennsylvania heading into the fall campaign, including an enormous boost in Democratic voter registration across the state - much of it fueled by excitement over April's Democratic primary fight between Obama and Sen. Hillary Rodham Clinton. As of Monday, there were 4.2 million registered Democrats in the state to the Republicans' 3.1 million.

And the latest Quinnipiac University poll shows Obama with a 7-point lead in Pennsylvania.

Democratic strategist Neil Oxman said that while Pennsylvania remained a key swing state, Obama was in a much more comfortable position here than in other battleground states, such as Ohio and Michigan.

Oxman said he believed that if Obama wins the state's southeastern counties and Pittsburgh, and does "respectably" with voters in Western Pennsylvania counties, "he'll win Pennsylvania by a very comfortable margin."

But Western Pennsylvania could pose a problem for Obama, analysts say.

During April's primary, Clinton commanded wide margins among Western Pennsylvania's predominantly white working-class voters. That is the segment of the state's voting populace that McCain will make herculean efforts to win over.

Since becoming the presumptive Republican nominee, McCain has made almost half of his visits to Pennsylvania in the western part of the state.

"McCain sees all kinds of weakness for Obama there," said Chris Borick, a professor of political science at Muhlenberg College.

Sabato, director of the the University of Virginia Center for Politics, said the issue of race was the silent unknown that also could influence how well Obama did in Pennsylvania.

"It shouldn't be a legitimate issue," he said, "but we all know it's in the closet."

Inquirer staff writer Thomas Fitzgerald contributed to this article.