Is Ryan ready to be president?

President Ryan. Try that on for size.

If the Republican ticket wins in November, Vice President Paul Ryan would be the proverbial heartbeat away from the presidency.

While former Alaska Gov. Sarah Palin faced a storm of criticism about her qualifications to succeed to the White House when John McCain picked her as his running mate in 2008, there has been virtually no debate in this year’s campaign about whether Ryan is ready to be president.

Presidential nominee Mitt Romney appears to be in good health and if he is elected there is no reason to think that his running mate would succeed to the presidency during his term of office.

There hardly ever is. And yet it has happened more than 20% of the time, with nine vice presidents out of our 44 presidents succeeding to the White House in the wake of death or resignation of the president.

In 2008, critics attacked Palin as not being ready for the presidency when gaps in her knowledge of world affairs and general lack of political experience became evident.

Commentator Bill Kristol and other conservatives had championed the charismatic Alaska governor for the vice presidential nomination because they thought she would bring “energy” to the campaign.

And it was Kristol who led the push this year to make Ryan the vice presidential nominee – again to tap into that youthful conservative energy. Kristol, in fact, would have preferred Ryan at the top of the ticket with Florida Sen. Marco Rubio as his running mate.

Conventional wisdom has it that vice presidential candidates don’t really tip the scales in voters’ choice of president. And yet the VP selection seems to be an important part of the mix and Romney’s choice of Ryan has generated a lot of debate and discussion, not least because of the Wisconsin congressman’s reputation as a fire-breathing fiscal hawk.

But Ryan has run into credibility problems, in weighty matters such as the inscrutable math of his budget proposals to less important questions like misstating his time running a marathon or staging a completely artificial photo-op washing pots in a soup kitchen.

Some of this may seem harmless, but when you’re the punchline for comedians – one joke last week said that after the 90-minute vice presidential debate, Ryan would claim he did it in 60 minutes – it can distract from your more serious message.

Peppering your convention speech with numerous fuzzy statements that require clarification from fact-checkers can also distract from your message.

But the main distraction from Ryan’s message of fiscal conservatism in this campaign has been his shyness in talking about it in any detail.

Remember that famous debate on Medicare that Ryan declared he welcomed and wanted to have and would win? Now he rarely brings up his plan for voucherizing Medicare and when asked just retreats into a mantra that nothing will change for those currently in or near retirement and his plan will “save” Medicare for the next generation.

None of that necessarily disqualifies him to run as vice president. But does it give voters confidence that he would be suitable as president if the need should arise?

It’s not something we like to talk about because it generally means a calamity of some sort has occurred. And yet, critics didn’t hesitate to fault Palin for being unsuitable for the White House.

Romney himself has no doubts about Ryan's readiness for the presidency. He told CBS's "60 Minutes" when he announced Ryan's selection in August that it was his first priority in choosing a running mate and that the Wisconsin congressman has the judgment, character and capacity to become president if necessary.

Perhaps because a seven-term congressman seems prima facie more qualified than a half-term governor, there has been little questioning of Ryan’s readiness for office, even though he would be one of the youngest vice presidents ever.

Ryan, who would be nine days shy of his 43^rd birthday when he took office on Jan. 20, would rank as sixth youngest behind John Calhoun, who took office 14 days before he turned 43.

Youth, however, is not in itself a disqualification. Richard Nixon, after all, was second youngest vice president at 40 and Dan Quayle third youngest at 41 when they took office in 1953 and 1989 respectively.

In recent times, vice presidential candidates have generally either won statewide office or served in a high administration position. George H.W. Bush, for instance, ran for vice president after serving as director of Central Intelligence, and Dick Cheney was defense secretary during the first Gulf war.

Those who ran directly from the House of Representatives have not been on successful tickets – New York Congressman William Miller was Barry Goldwater’s running mate in 1964 and Geraldine Ferraro, also from New York, ran with Walter Mondale in 1984.

Voters seem to prefer that candidates for president or vice president have a wider purview than that gained by a congressman representing his or her district. Also, standing the test of a statewide contest is valuable seasoning for a national campaign.

Although Ryan had clearly cribbed well for foreign policy questions in last week’s debate, he was left floundering against Vice President Joe Biden, a former chairman of the Senate Foreign Relations Committee, in a lengthy discussion of Afghanistan.

In general, some of Ryan’s missteps in the campaign could be attributed to lack of experience outside the cosseted environments of Congress and his Wisconsin district. While not exactly jejune, he often does not come across as having the maturity or gravitas to take charge in the White House.

Of course, if the Romney-Ryan ticket wins the day Nov. 6, he probably would never have to. Unless he does.