If tonight’s reporting is correct, and if Mitt Romney really does announce at 9 A.M. Saturday that Congressman Paul Ryan of Wisconsin is his running mate, Romney has made the most daring decision of his political career.

After spending weeks looking into Ryan’s history for The New Yorker, visiting his home town, and interviewing him twice, I am genuinely surprised that Romney chose him. First, let’s tally the risks of a Ryan pick.

For one thing, Ryan has no significant private-sector experience. Besides summer jobs working at McDonald’s or at his family’s construction company, or waiting tables as a young Washington staffer, Ryan has none of the business-world experience Romney frequently touts as essential for governing. In the run-up to his first campaign for Congress, in 1998, that gap was enough of a concern for Ryan that he briefly became a “marketing consultant” at the family business, an obvious bit of résumé puffing.

But Ryan’s Washington experience is also light, at least for a potential President—which, after all, is the main job description of a Vice-President. Ryan has worked as a think-tank staffer and Congressman, but he’s never been in charge of a large organization, and he has little experience with foreign policy. Given how Sarah Palin was criticized for her lack of such experience, I’m surprised that Romney would pick someone whose ability to immediately step into the top job is open to question.

And the experience that Ryan does have is not exactly what voters are clamoring for at the moment. The bulk of Ryan’s House career coincided with the Presidency of George W. Bush, during which he was a reliable vote for many Bush policies that have not aged well: Medicare Part D; the Iraq War; and the Troubled Asset Relief Program. Ryan told me that voting for all of that spending, which added trillions to the deficit, made him “miserable,” but he’ll need a better explanation in his October debate with Joe Biden.

Presumably, Romney’s main reason for picking Ryan is not his early deficit-busting record but his more recent rise to celebrity as a crusading policy wonk determined to tame the federal government. Romney, who has been extremely vague about what he would do if elected, will now own Paul Ryan’s ideas, which include privatizing Social Security, turning Medicare into a voucher program, bloc-granting and drastically cutting Medicaid, and reducing discretionary spending to levels that would affect every popular government program. This Ryan agenda will now fill the vacuum created by Romney’s unwillingness to lay out the specifics of his own plan. Even before this (apparent) announcement, Democrats were planning on tying Romney to Ryan’s policy platform. Now Romney has done it for them.

So what’s the potential upside? Romney seems to have realized that his spring and summer strategies have been a failure. Since winning the nomination, Romney’s plan has been to turn the election into a simple referendum on Barack Obama. With the ailing economy, Romney believed, he needed to do little more than stand around and wait for voters to sour on the incumbent. When they did, Romney would be there as the default alternative. In recent weeks, as Romney’s favorable ratings declined, some encouraging economic news dribbled out, and Obama’s poll numbers ticked up, a loud faction of Republicans began pointing out that Romney’s theory of the campaign was wrong. Their argument was that Romney needed to turn the race into more of an ideological debate. He needed, these Republicans said, to embrace a bold policy agenda that would dramatically contrast with Obama’s. Nobody made this case more loudly than Paul Ryan. Presidential candidates shouldn’t “run on vague platitudes and generalities,” he told me in one interview. “I want a full-throated defense for an alternative agenda that fixes the country’s problems,” he said in another.

Romney’s choice of Ryan will undoubtedly be criticized as capitulation to the right, and this pick does seem to demonstrate that Romney is not able or willing to distance himself from the base of his party. But the good thing about the Ryan pick is that the Presidential campaign will instantly turn into a very clear choice between two distinct ideologies that genuinely reflect the core beliefs of the two parties. And in that sense, Romney’s choice of Paul Ryan is good news for voters.

For more on Paul Ryan and the announcement of his place on the Republican ticket, see Ryan Lizza’s full Profile of the Wisconsin Congressman, James Surowiecki on Ryan’s budget, John Cassidy on Romney’s “Let’s talk about Ryan, not me” strategy, Jane Mayer on Ryan’s affection for libertarian hero Ayn Rand, Alex Koppelman on why the pick had to be Ryan or someone like him, and Amy Davidson on the first speeches that Romney and Ryan gave as running mates. And bookmark The Political Scene, the hub for all of The New Yorker’s coverage of the 2012 campaign.

Illustration by Jorge Arevalo