As he prepares to give the most important speech of his career tonight as the Republican nominee for vice president and introduce himself to millions of Americans, the 42-year-old congressman from Wisconsin must sell voters on a different proposition: his own readiness to become president of the United States.
"Can he step in and do the job? That's really the only thing that matters," said Romney pollster Neil Newhouse.
For voters, it's a calculus that takes into account not only his age and experience, but the intangibles of his confidence, competence and preparedness.
The question bedeviled Geraldine Ferraro in 1984, Dan Quayle in 1988 and Sarah Palin in 2008.
As the freshest face among the four men atop the Republican and Democratic tickets, Ryan has the best opportunity to make a first impression - but the shortest time to prepare. He's been a national candidate for 18 days.
"You have to pass a threshold plausibility test," said Joel Goldstein, a law professor at St. Louis University and leading expert on the vice presidency. "Do they have the credentials or the résumé significant enough that you can see him as president?"
Ryan's years in the House of Representatives provide the foundation for that résumé. After serving as a congressional staffer, he first ran for Congress at age 28 and has served 14 years. He catapulted past more senior members to become chairman of the House Budget Committee in 2011 on the strength of his reputation as a budget "wonk."
From that position, Ryan authored the Republican budget plan passed by the House earlier this year that included deep spending cuts and a restructuring of Medicare that has become a flashpoint of debate in the political campaign season.
"From that standpoint, he's at least in the ballpark" of the kind of experience voters expect a vice president to have, Goldstein said.
President Obama- then a first-term senator after eight years in the Illinois Legislature- faced similar questions about his readiness in 2008, a fact Ryan now reminds voters of.
"Obviously, I have a lot more experience than Barack Obama did when he became president," Ryan told Fox News' Sean Hannity last week.
Republicans most hope to contrast Ryan's experience with Vice President Biden's. Though a seven-term senator, two-time presidential candidate and sitting vice president, Biden's extemporaneous speech is often looser than Ryan's carefully measured statements.
Ryan "says what he thinks - but not in the same way Joe Biden does. I think Paul has been relatively gaffe-free," Newhouse said. "I think you're defined more by your mistakes than by your accomplishments or your record, and that's where Joe Biden is lacking."
A USA TODAY/Gallup Poll taken right after Ryan was named showed 48% of Americans thought him qualified to serve as president - higher than Quayle or Palin, the two modern candidates closest to him in age, but lower than prior vice presidential nominees.
'The Bentsen test'
As Ryan begins introducing himself to a national audience tonight, Goldstein says his most important goal is addressing the "Bentsen test."
In 1988, Quayle - a year younger than Ryan is now - said his 12 years in Congress meant he had "as much experience in the Congress as Jack Kennedy did when he sought the presidency." That led Democratic nomineeLloyd Bentsen to famously blistering retort in a debate, "Senator, you're no Jack Kennedy."
The lesson, Goldstein said, is that the length of experience isn't as important as how voters judge the quality. And when Ryan debates Biden on Oct. 11, he will, like Quayle, be facing an adversary decades older, who occupied senior leadership positions in government while his younger opponent was still in school.
Ryan's backers say tenure isn't everything.
"Twenty-five years of experience can be 25 years of making the same mistakes over again," said former Education secretary William Bennett, a mentor of Ryan's at the conservative group Empower America.
"Readiness is all," he said, quoting Hamlet. "And I think he's ready."
"The Budget Committee is pretty big time," Bennett said. "He knows what a full day is. He's agile and smart and prepared. Whether you're a Republican or a Democrat or for him or against him, you know that he has serious ideas and he means business."
Democrats have picked at pieces of Ryan's record, noting his lack of significant foreign-policy experience and his failure to push a legislative agenda through a mostly divided Congress, but the thrust of their attacks has been more that Ryan is a conservative extremist than a lightweight.
A background document on Ryan circulated by the Obama campaign contains six items attacking his foreign-policy pedigree - and 26 on abortion and women's rights.
It's Ryan's positions on federal spending, taxes and fiscal policy that have made him the Republican gold standard.
Rep. Chris Van Hollen, D-Md., is probably the Democrat who knows Ryan best - so much so that he's been tapped to play Ryan during Biden's preparations for the vice presidential debate.
As the ranking Democrat on Ryan's Budget Committee, he and Ryan have enjoyed an unusually collegial relationship. He said Ryan is easy to like but difficult to work with. "There's a big difference between collegiality and a willingness to compromise on policy issues."
Attacks on Ryan started within minutes of him being named to the ticket and have helped to polarize public opinion on him. Heading into the convention, Ryan was the most controversial running mate in a generation: 38% of Americans view him favorably, but 36% have an unfavorable view, according to a USA TODAY/Gallup Poll. That's the highest unfavorable rating at the convention of any non-incumbent vice presidential candidate in the 20-year history of the poll.
Ryan doesn't have the typical vice presidential résumé. Indeed, parties have rarely looked to the House to fill out a ticket. The last sitting House member nominated as vice president was Ferraro, a three-term New York Democrat picked by Walter Mondale in 1984 as the first woman to run on a major national ticket. The last Republican was Rep.William Miller, R-N.Y., Barry Goldwater's running mate in 1964. Both lost.
Yet Ryan's rise is proof of how influential House Republicans have become to the GOP, said Guy Harrison, the chief strategist for the House Republicans' campaign operation.
"We have a House candidate as our vice president. I think that shows you a pretty good idea of how House Republicans are going to be involved in the presidential race," he said. "We take a lot of pride in Paul Ryan."
Is the jump from the House to a national ticket too high a hurdle? Ryan has never run for statewide office, and each House member represents only about 0.2% of the nation's population.
Senators tend to get much of the attention on Sunday talk shows, especially on foreign policy, but Ryan is a veteran of the TV circuit, getting more Sunday morning face time this year than any member of Congress but Sen. John McCain, R-Ariz., according to a database of appearances maintained by the Capitol Hill newspaper Roll Call.
The foreign policy question
Obama and Biden both earned their foreign-policy stripes by serving on the Senate Foreign Relations Committee, a résumé gap with the GOP ticket that Democrats hope to exploit.
"Whether it's the Cold War or the age of terror, to not have someone with national security credentials is unusual," Goldstein said.
In addressing his foreign-policy experience, Ryan has noted his votes in favor of wars in Afghanistan and Iraq - "that's a vote you take very seriously, very solemnly," he said inNew Hampshire last week - and his work on the budget, which proposes maintaining robust defense spending.
While addressing the Alexander Hamilton Society, a neoconservative group, last year, Ryan tied the nation's fiscal underpinnings to foreign policy and national defense: "Our fiscal policy and our foreign policy are on a collision course; and if we fail to put our budget on a sustainable path, then we are choosing decline as a world power."
Ryan also has noted his congressional travel has focused on the Middle East, including trips to Afghanistan, Iraq, Saudi Arabia and the United Arab Emirates.
Republican National Committee Chairman Reince Priebus shrugs off questions about whether Ryan is ready, telling USA TODAY that Ryan has been preparing for this moment his entire career.
"He tries to win the mission every day. He works hard, he waits on God's timing and generally good things happen," he said. "I've seen Paul on big stages. There is no doubt that Paul will hit it out of the park."
Contributing: Jim Norman in McLean, Va.; Susan Davis and Jackie Kucinich in Tampa