Republican Mitt Romney reset the race for the presidency as a battle over the size and scope of the federal government Saturday, choosing as his running mate Rep. Paul Ryan of Wisconsin, the architect of the GOP’s plans to slash spending and overhaul Medicare.

In a risky and surprising move to give his campaign a jolt of momentum, Romney chose the 42-year-old congressman over several contenders considered safer bets. The selection seemed destined to shift the tone of a campaign that has become mired in petty squabbles and force a debate over how to tackle the nation’s fiscal challenges.

In tapping the conservative star, Romney inextricably tied himself to Ryan’s controversial vision for erasing the country’s red ink — a revamp of Medicare, deep spending cuts and a restructured tax code that would lower all rates, even for the wealthy.

Romney announced his selection in dramatic fashion Saturday morning, with each man stepping down the deck of the USS Wisconsin — a World War II-era battleship named for Ryan’s home state — to the soundtrack of the movie “Air Force One.” And at a trio of packed rallies across Virginia, Romney praised his new partner as a servant of America’s working classes and a citizen of integrity, character and vision.

“He’s never been content to simply curse the darkness,” Romney said of Ryan. “He’d rather light candles.”

Before a cheering crowd of more than 1,500 here, Ryan introduced himself by saying the ticket offers a brand of candid leadership missing under President Obama.

“The commitment Mitt Romney and I make to you is this: We won’t duck the tough issues; we will lead,” Ryan said. “We won’t blame others; we will take responsibility. And we won’t replace our founding principles; we will reapply them.”

The two then loaded onto Romney’s campaign bus with their wives and families to travel north through this important swing state, showing in their first hours together an easy rapport that reflected the chemistry that had impressed Romney’s aides from the time the two first campaigned together in April.

Ryan was a pick for a candidate in need of a jolt; recent polls had shown Obama with a small but steady lead over Romney in key swing states.

The selection achieved the rare feat of pleasing leaders in both parties. Top conservatives said Ryan would energize the party’s base and offer an articulate and robust defense of smaller government. Democrats said they were equally convinced voters will reject Ryan’s prescription for deficit reduction as too harsh. Obama campaign manager Jim Messina quickly called the Ryan budget proposals “radical” and said they would ensure “budget-busting tax cuts for the wealthy, while placing greater burdens on the middle class and seniors.”

Democrats immediately blasted supporters with fundraising notices about the pick. “Yeah — that Paul Ryan. The architect of the Republican plan to kill Medicare,” began one from the Democratic Congressional Campaign Committee.

Romney promised that Ryan would be a candidate who “appeals to the better angels of the American people” and could help lift the national dialogue.

But even the signal moment of introducing the Republican ticket did not escape the negativity that has defined the race so far. Both Romney and Ryan wove a healthy dose of attacks on Obama throughout their remarks. In Manassas, Ryan told a sprawling crowd that Obama has led the country on a path toward “debt, doubt, despair and decline.”

Although Romney’s pick does not replicate the shock of Arizona Sen. John McCain’s choice of Alaska Gov. Sarah Palin four years ago, Ryan is nonetheless a splashier choice than other contenders who had been considered safer options.

The campaign went to extraordinary lengths to ensure the news did not leak, even as conservative commentators waged a public campaign in recent days to encourage a Ryan pick.

Romney had largely settled on Ryan by the time he returned home from his foreign trip on Aug. 1, aides said. After meeting with nine of his top advisers for a final “gut check,” he called Ryan and invited him to a meeting in Boston four days later, said Romney adviser Beth Myers, who ran the vice presidential search.

Ryan — wearing a baseball hat and sunglasses — flew undetected from Chicago to Hartford, Conn., where Myers’s son, Curt, drove him in a rented sport-utility vehicle back to the family home in Massachusetts. After lunch with the Myerses, Ryan met alone with Romney, who asked him to be his running mate. Ryan accepted.

The initial rollout plan was for Friday in New Hampshire, but when the memorial for a shooting at a Sikh temple in Ryan’s district was scheduled for that day, the campaign moved the announcement to Saturday. After the memorial, Ryan returned home — then sneaked out the back door, through the woods and out to a waiting car driven by an aide.

After a chartered flight from Waukegan, Ill., to Elizabeth City, N.C., Ryan was whisked to a hotel and reunited with his family. On Saturday, a Secret Service detail ferried him to the USS Wisconsin.

“It’s gone from the surreal to the real,” Ryan said aboard Romney’s campaign plane Saturday night. “By the time we met in person, I kind of knew it was going to happen. I was very humbled. It was the biggest honor I’ve ever been given in my life.”

More than two decades Romney’s junior, Ryan brings a youthfulness and policy expertise to the ticket. The father of three school-age children, he is the same age as Romney’s eldest son — and with his similar shock of dark hair, has been jokingly referred to as the nominee’s “sixth son.”

Elevated to chairman of the Budget Committee with the Republican takeover of the House in the 2010 midterm elections, Ryan authored a budget proposal that called for reducing deficits by dramatically shrinking spending. Notably, the plan called for reshaping Medicare into a government voucher that would allow seniors to purchase private insurance, an attempt to curb the rising costs of the entitlement program.

Ryan argues his proposal would salvage Medicare by altering its unsustainable growth trajectory. Democrats say the proposal would save the government money by shifting the costs of care onto seniors. Statements from Democratic leaders Saturday accused Romney of endorsing a plan to shred the social safety net and end Medicare.

That idea will now play a key role in critical states with large retiree populations, most notably Florida, where Romney is scheduled to campaign on Monday. On Sunday, the pair head to North Carolina, then to a “homecoming” rally for Ryan in suburban Milwaukee. From there, they are scheduled to part ways, Ryan to the Iowa State Fair and Romney to continue his bus tour.

Not since 1984, when Geraldine Ferraro was chosen as Walter Mondale’s running mate on the Democratic side, has a House member been on a major party’s ticket. And it has been 80 years since a member of the House has been on a ticket that won. The ticket makes history in other ways: It will be the first in U.S. history not to include a Protestant. Ryan, a Catholic, joins the first Mormon.

The pick was cheered by members of the party’s conservative wing, and both of the last two Republican presidential nominees voiced immediate support for the choice, with McCain calling the Romney-Ryan ticket “the strongest team to return America to prosperity” and former president George W. Bush terming Ryan a “strong pick.”

For voters unfamiliar with the budget debate, Ryan will probably start out as an unknown. According to recent polls, reviews of Ryan tilt positive in Wisconsin and nationally, but not overwhelmingly so. Large numbers, even in his home state, do not know enough about him to rate him. In a CNN poll last week, Sen. Marco Rubio (R-Fla.) was the top pick as the vice presidential nominee among Republicans and GOP-leaning independents.

Ryan could use his charm to offer digs at Obama not weighted with negativity, although he has had a tense relationship with the president. Not long after Ryan unveiled his budget framework in 2011, Obama invited the congressman to attend what the White House billed as a major economic address at George Washington University.

Obama proceeded to stun Ryan by eviscerating his plan while the congressman sat silently in the front row. Obama said there was “nothing serious” about reducing the deficit while cutting taxes for millionaires and billionaires and nothing “courageous” about asking for sacrifices only from those who cannot afford them. Ryan complained later that Obama had called him “un-American.”

Karen Tumulty, Lori Montgomery and Dan Balz in Washington contributed to this report.