Thursday night's convention speech, the symbolic opening of President Obama's general-election effort, represents an important moment in this year's campaign.
But no matter what the President says, and no matter how well he says it, history likely will record that the 2012 presidential race was transformed not in September but in August -- and not by anything that happened at the Republican National Convention in Tampa last week.
This is no longer a campaign of Mr. Obama against Mitt Romney. It's a campaign of Mr. Obama against Paul Ryan.
Not in a generation -- perhaps not ever -- has a vice-presidential nominee had such a transformative effect on a national political ticket.
By selecting Mr. Ryan, Mr. Romney did more than signal what kind of administration Republicans would conduct if they win the White House in November.
He did more than catapult Mr. Ryan, until now a cult hero to a tiny enclave of think-tank conservatives, into a national figure and, barring a catastrophic faux pas, a presidential candidate in his own right four or eight years from now.
Mr. Romney's choice transformed his presidential campaign into a conversation about Mr. Ryan's personality, character, and ideas.
Some running mates have delivered fresh credibility and excitement to a campaign. That's what Sen. Edmund Muskie did in 1968, when then-Vice President Hubert Humphrey added him to the Democratic ticket, much as Sen. Joseph Lieberman did when he joined Al Gore as his vice-presidential nominee in 2000.
Both tickets lost close races. Both running mates faltered early in their own, separate presidential campaigns four years later.
But Mr. Ryan did something that neither Mr. Muskie nor Mr. Lieberman did: He added a new ideological cast to his ticket.
Mr. Ryan stirs something deep in the modern Republican Party. For the past six elections, the GOP has had a rebellious, muscular conservative undertone.
Each time, it has nominated for president a conventional Republican, steeped in establishment values and possessing conventional qualifications -- a vice president, two incumbent presidents, two high-profile senators who had both lost earlier nomination fights. Mush that together and you have Richard Nixon, who, remarkably, qualifies on all counts.
Mr. Romney is conventional as well. Like George W. Bush, he served as governor of an important state. Like both Presidents Bush, he was reared by a prominent political father.
But Mr. Romney's running mate is a House member, which is unusual but not unprecedented. Mr. Ryan also is the author of the principal Republican budget proposal, which takes dead aim at the spending and entitlement programs that are like the weather: Everyone talks about them, but nobody does anything about them.
Much of last week's GOP convention was designed to burnish the image of Mr. Romney, to add human dimensions to his profile, to portray him as an experienced job creator who can do what Mr. Obama has failed to do: trim unemployment substantially and get the economy moving.
Seeking to make the public comfortable with an alternative to the incumbent is standard fare for a challenger's convention.
But what is different in 2012 is that the table-setting for the fall campaign has been altered. Nobody is talking much about Mr. Romney's ideas. People are talking about Mr. Ryan's. Nobody is saying there is Republican excitement over Mr. Romney. People are talking about Mr. Ryan.
This week, the Democrats will spend plenty of time demonizing Mr. Romney. They'll say he is an outsourcer, a heartless private-equity wizard whose eyes turn to the bottom line and away from the human costs of efficiency, and a bloodless executive lacking the common touch.
But the ideas the Democrats will demonize won't be Mr. Romney's, they'll be Mr. Ryan's. He's the guy who makes Republican hearts flutter -- and who gives Democrats heartburn.
Since spring, the political class has wondered what might happen when Mr. Obama and Mr. Romney meet for a debate. But that may not be the main event.
The real fun will occur Oct 11. That's when Vice President Joe Biden and Mr. Ryan face off in front of a national audience. Skip it and you'll likely miss the biggest moment of the 2012 campaign.
David Shribman is executive editor of the Pittsburgh Post-Gazette.
Contact him at: firstname.lastname@example.org