Republicans call him the only adult in the room willing to deal with tough issues. Democrats say his ideas would decimate Medicare and the middle class.

Rep. Paul Ryan, the Wisconsin Republican tapped on Saturday as Mitt Romney's vice presidential running mate, is a bold, potentially polarizing choice who will excite Romney's conservative base and bring into deeper relief a campaign debate over two vastly different visions for the economy and government.

The fourth child of a Janesville, Wis., attorney and a stay-at-home mom, Ryan, 42, offers a fresh face, small-town roots and strong conservative credentials to a Romney campaign often perceived as elitist and establishment driven. As a vice presidential candidate, Ryan also brings an authoritative voice for smaller government that could prove either a boon or blow to Romney's prospects.

Paul Ryan's career
1992: Miami University of Ohio, Bachelor's Degree in economics and political science,
1992: Aide to Sen. Bob Kasten, R-Wis.
1992-95: Speechwriter, economic analyst at now-defunct Empower America
1995-97: Legislative director to Sen. Sam Brownback, R-Kan.
1998: Elected to Congress, succeeding Rep. Mark Neumann, R-Wis.
2010: Releases "Roadmap for America's Future," a plan for overhauling Medicare and simplifying tax code
2011: Named chairman of the House Budget Committee
2012: Becomes Mitt Romney's vice presidential running mate

Ryan, chairman of the House Budget Committee, has established himself as the GOP's top fiscal wonk. His deficit reduction plan, "The Path to Prosperity," has become the benchmark against which the conservative credentials of prospective candidates are judged.

Ryan's blueprint advocates drastic spending cuts, but also tackles entitlement reforms head-on, making it an easy target for Democrats, who have charged that he wants to end Medicare. Just moments after Romney announced Ryan as his choice, President Obama was already claiming in a fundraising email to supporters that they needed to band together to protect the popular entitlement.

"It reminds me of 1995 when we went after [former House Speaker Newt Gingrich] and said, 'These guys are going to destroy Medicare and Medicaid and I'm the only thing between you and that assault,'" said Patrick Griffin, a legislative adviser to former President Bill Clinton. "Democrats were already trying to hang this budget on Mitt Romney. Now Mitt Romney has put on the jacket. It's playing into the opposition strategy."

But Ryan also energizes conservatives, who until now have been wary of Romney. Former Sen. Rick Santorum, a Romney rival for the nomination who won over the conservative voters who eluded Romney, called Ryan "an outstanding choice" who "demonstrates Gov. Romney's commitment to returning fiscal sanity back to Washington."

Ryan, a fifth-generation Wisconsinite who entered Congress in 1999, is the first Generation X-er on a presidential ticket. His seven terms in Congress balances Romney's outside status while still appealing to Midwestern voters in tossup states like Ohio and Iowa.

"You wonder whether they were concerned about Romney's affluence and wanted someone with more of a common person appeal," said Joel Goldstein, author of "The Modern American Vice Presidency." "Romney went out of his way to mention Janesville, Wis. That's going to be what [Vice President Joe Biden's hometown of] Scranton is to the Democratic ticket."

Republicans hope Ryan could even put on play Wisconsin, where Obama had a cushy lead in the polls, though a Marquette Law School poll shows that many Wisconsinites don't really know the congressman from the state's largely rural First District.

"As citizens come to know what both campaigns are saying, will they conclude with a more positive view of Ryan or a negative view of Ryan?" said Charles Franklin, director of the Marquette Law School Poll. "The issue here for Ryan is not the personal side, it's the Ryan budget."