Romney appears to have heard the chorus of party conservatives and deficit hawks who were clamoring for Ryan on the ticket. Before Saturday’s announcement, many of those voices had worried about Romney’s political compass and feared that he might become more moderate in the general election.

“Paul Ryan is a solid conservative leader whose conservatism is indivisible,” said Ken Blackwell, a former Ohio secretary of state who’s a board member for the fiscally conservative Club for Growth and the National Taxpayers Union. “His commitment to advancing American exceptionalism is rooted in his embrace of our pro-life, pro-family, pro-growth, pro-defense traditions.”

But Ryan’s rise to the vice-presidential nomination carries risks. He galvanizes Republicans, but he also energizes Democrats, who despise his budget plan. Democrats said they looked forward to using him as a foil to show what’s wrong with Romney’s and the Republican Party’s policies and rhetoric.

Several Democrats eagerly noted Saturday that Ryan likes to talk tough about deficits and the evils of government entitlement programs yet he voted for the Medicare Part D prescription-drug expansion in 2003 and the Troubled Asset Relief Program in 2008, which bailed out some of America’s biggest financial institutions.

“Now with Congressman Ryan on the ticket, House Republicans face the one thing they hoped to avoid: a national debate on their budget that puts millionaires first and Medicare and the middle class last,” said Rep. Steve Israel of New York, the chairman of the Democratic Congressional Campaign Committee.

The general public hasn’t embraced Ryan’s plan to overhaul Medicare, though it’s popular among many conservatives. A CNN/Opinion Research Corp. poll in June 2011 found that 58 percent of Americans opposed the Republican Medicare plan while 35 percent said they supported the measure. Among senior citizens, who compose a sizable voting bloc in swing states such as Florida, 74 percent opposed the Republican-proposed Medicare plan.

Another potential problem the Romney-Ryan ticket faces is a lack of foreign-policy expertise. Recent successful presidential candidates with few foreign-policy credentials have balanced their tickets with running mates known for their expertise in international affairs, intelligence or defense expertise: Dick Cheney for President George W. Bush; Al Gore for President Bill Clinton and George H.W. Bush for President Ronald Reagan.

“They’ve obviously made the determination that the election is about the economy,” Walch said.

Romney is betting that he and Ryan can buck history and have a member of the House become vice president. That happened only three times in the 20th century, Walch said: James Sherman, a House member from New York, was William Howard Taft’s vice president; John Nance Garner, a Texan and House speaker, was elected Franklin D. Roosevelt’s vice president; and Gerald Ford was appointed vice president after Spiro Agnew resigned as Richard Nixon’s number two after pleading no contest to tax evasion.

“Serving as a congressman generally does not give you a boost on the national ticket,” Walch said. “Recent vice-presidential nominees who came directly from the House, such as Geraldine Ferraro, are rare indeed.”