Republican Mitt Romney is betting that his selection of Rep. Paul Ryan as a running mate will rally his conservative base in a nip-and-tuck election. President Obama and his Democratic allies are counting on its having the same effect on their side.

No sooner had Romney introduced the Wisconsin congressman as his vice presidential choice in front of a battleship in Norfolk, Va. on Saturday, than the Obama campaign went to war, painting Ryan as a “radical” ideologue whose extreme views would lead to a reprise of the “same, catastrophic mistakes” of the George W. Bush era.

Democrats in Congress and liberal activist groups piled on, denouncing Ryan and sending out frantic fundraising pitches that played off fears of a GOP administration to solicit donations in hopes of matching the Republican cash grab in the wake of the announcement.

Beneath the fierce response was a sense of delight among Democrats that they got the vice presidential candidate they wanted in Ryan, a staunch fiscal conservative. For months, the Obama campaign has been trying to tie Romney to Ryan’s Republican House budget proposal, which the president in April called “social Darwinism” that would pit the poor against the wealthy.

Democrats believe Ryan’s ideological views will turn off moderate voters and drive liberals to the polls, especially in Florida, a critical swing state where Obama, in two appearances last month, vilified the congressman’s proposal to partially privatize Medicare. In this way, Democrats say, Ryan provides a natural foil for the president, who has framed the election as a choice between sharply contrasting visions that could fundamentally reshape the nation.

In a statement, Obama campaign manager Jim Messina said that Romney has “chosen a leader of the House Republicans who shares his commitment to the flawed theory that new budget-busting tax cuts for the wealthy, while placing greater burdens on the middle class and seniors, will somehow deliver a stronger economy.”

Obama did not respond to questions shouted by reporters as he left the White House on Saturday afternoon for a trip to Chicago, where he is to attend four fundraising events Sunday. The president’s campaign said that Vice President Biden called Ryan to welcome him to the race, saying he “looked forward to engaging him on the clear choice voters face this November.” The two are scheduled to debate Oct. 11 in Danville, Ky.

Yet the Obama campaign’s rapid response showed it was well-prepared for Ryan. The president’s operatives posted a Web video denouncing Ryan as the “mastermind behind the extreme GOP budget plan,” and they added a new page to the campaign’s Web site mocking the Romney-Ryan partnership as the “Go Back Team,” riffing off the Romney campaign’s labeling of the ticket as “America’s Comeback Team.”

In a fundraising e-mail, Messina wrote: “Our job is to make sure Americans know the truth about what Romney’s choice says about him as a candidate and leader.”

The president’s Democratic allies echoed the campaign’s criticism of Ryan. Senate Majority Leader Harry Reid (D-Nev.) said that Romney’s choice “demonstrates that catering to the tea party and the far right is more important to him that standing up for the middle class.”

Bill Burton of Priorities USA Action, a super PAC that supports Obama, vowed that his organization would shift the focus of its television ads--which have focused exclusively on Romney’s tenure at Bain Capital two decades ago--to Ryan and his budget.

“He was the one nominee who could actually do damage to the ticket,” Burton said, acknowledging that he was surprised by Romney’s choice. “Everybody else was fairly neutral. No doubt he will fire up conservatives, but he also comes with so many liabilities from his budget that Romney will come to think he made a sizable mistake attaching himself so closely to Paul Ryan.”

Democrats running in statewide races also jumped on the anti-Ryan bandwagon. The Democratic Congressional Campaign Committee warned in a fundraising e-mail that Ryan will “open the floodgates” of donations from conservatives to Romney.

“Don’t let them use this moment to beat us. Help us keep pace,” the e-mail pleaded.

Tim Kaine, who is engaged in a fierce battle against Republican — and fellow former Virginia governor — George Allen for a Senate seat from that state, sent a fundraising e-mail that tied Allen to the Ryan budget and added: “Even Newt Gingrich called it ‘right-wing social engineering.’ 

Obama has centered his campaign on an appeal to the middle class, emphasizing his belief that the federal government should play a role in investing in critical public needs such as infrastructure, education and health care. Republicans have attacked him for allowing the deficit to grow while pumping money into government programs, such as the stimulus package and the health-care reform bill, that have failed to jump-start the economy.

In a speech in April to the Associated Press, Obama used the Ryan plan as a metaphor for a GOP vision for the country that he said is “antithetical to our entire history” as a land that promises an upward path for the middle class.

“It’s a Trojan horse. Disguised as deficit reduction plan, it’s really an attempt to impose a radical vision on our country. It’s nothing but thinly veiled social Darwinism,” Obama said then. “It’s a prescription for decline.”

By Saturday evening, Obama had arrived in Chicago, his Marine One helicopter swooping into his home town and dropping him near Soldier Field. His motorcade then proceeded directly to his campaign headquarters, where the president huddled with senior staff members, plotting the next steps in a campaign entering a new phase.