"We're in a difficult and dangerous moment," Ryan said. "We are running out of time, and we can't afford four more years of this."
The son of a highway contractor, Ryan is a seven-term congressman who broke in as an aide to Jack Kemp and has spent most of his career within the confines of the Beltway. In the Capitol, he has often been a voice in the wilderness about confronting the rising cost of federal entitlements.
The choice was applauded by the GOP right, whose leaders had urged Romney in recent weeks to pick Ryan over safer choices on the short list of finalists, such as Sen. Rob Portman of Ohio and former Minnesota Gov. Tim Pawlenty. The move may help allay lingering doubts among some conservatives about Romney, who governed as a moderate in the Massachusetts State House.
Yet the announcement also fired up Democratic activists, who believe Ryan's plan to turn Medicare into a voucher program provides them an opportunity to win over seniors and boost their efforts to cast Romney, a wealthy former private equity executive, as hostile to the interests of the middle class.
President Obama's campaign manager, Jim Messina, swiftly labeled Ryan "the architect of the radical Republican House budget" proposal and said that plan "would end Medicare as we know it."
Democrats say they had been planning to try to link opponents in House races to the Ryan plan in any event - a tactic the GOP derisively calls "Mediscare." But the tactic has worked before - it helped Rep. Kathy Hochul (D., N.Y.) win a special election in an upstate district last year.
On that score, at least, Ryan's newly raised profile may make Democrats' task easier.
"It's a gift to Democrats running against vulnerable Republicans," said Mark Nevins, a partner in the Dover Strategy Group, who advises dozens of House candidates. "It takes an issue we were hoping would become high profile [Medicare] and makes it national."
Romney's decision changed the conversation immediately after months of Democratic assaults that painted him as an insensitive plutocrat. TV ads have battered Romney's record as head of a private-equity firm, arguing he became wealthy by slashing U.S. jobs and then, for a time, stashing some of his millions offshore.
Recent polls suggest those attacks have taken a toll, with Obama moving into a lead in key states and, in some national surveys, Romney's personal favorability ratings dropping.
Romney, in his Saturday remarks, sought to make a contrast with his attackers, saying he and Ryan were about pushing bold solutions.
"At a time when the president's campaign is taking American politics to new lows, we are going to do things differently," Romney said. "We are going to talk about aspirations and American ideals."
Indeed, for the day at least, the campaign seemed to rise above the name-calling and gusts of outrage over small issues for an argument that has bedeviled the country since before there was a country: What is the proper size and role of government?
"I think his selection truly sets the stage for the debate that America needs," said Colin Hanna, a former Chester County commissioner, social conservative activist, and early backer for former presidential candidate Rick Santorum.
"With Ryan now on the Romney ticket, we have a debate on limited government versus ever larger government in a more substantive way than we might have had with some of the other possible picks."
In addition to Portman and Pawlenty, Romney passed over Gov. Christie and Florida Sen. Marco Rubio. But some Republicans, while pleased with Ryan's gravitas, privately worried that he could be a drag in states such as Florida and Pennsylvania, with high percentages of retirees, many of whom rely on Medicare.
At least the choice knocked down a theme that has taken hold among many in politics: that Romney is too cautious - "meek," as one Northeastern GOP leader put it Saturday. Ryan was a bold choice, and caught many by surprise.
If Ryan succeeds, he'll be in a small group - House members who jumped to the vice presidency by election. The last was Texas' John Nance Garner, elected with Franklin Roosevelt in 1932. In 1984, Democrat Walter Mondale made New York's Geraldine Ferraro his running mate; they lost 49 states. Two decades earlier, William Miller was Barry Goldwater's GOP running mate in a landslide loss.
But no landslide is expected Nov. 6, and some Republicans predicted that Ryan's choice might actually improve the tone of a race that has been bitter and close.
Rep. Frank LoBiondo (R., N.J.), who said he and Ryan are "sort of gym rats together" at workouts, said his friend has a rare gift for synthesizing complicated policy issues so everyone can understand them.
LoBiondo said, "Paul Ryan has the ability to raise the caliber of the debate to being very substantive."
Contact Thomas Fitzgerald
at 215-854-2718 or tfitzgerald@ phillynews.com or @tomfitzgerald on Twitter. Read his blog, "The Big Tent," at www.philly.com/bigtent.
Staff writers Jeremy Roebuck and Jonathan Tamari contributed to this article.