The past five Republican vice-presidential candidates have all been quite conservative. But the four who preceded Representative Paul D. Ryan also had other strengths — and they were usually chosen in the hope of balancing out some perceived deficiency at the top of the ticket.

Dick Cheney was supposed to give gravitas and foreign policy experience to George W. Bush. Dan Quayle and Sarah Palin were supposed to provide youthful energy to the campaigns of George H. W. Bush and John McCain. (Mitt Romney has rarely been questioned for being “too old,” perhaps because he looks much younger than his 65 years.)

Bob Dole’s choice of Representative Jack Kemp of New York might be more closely comparable to Mr. Romney’s selection of Mr. Ryan, but it is not a perfect one. Mr. Kemp, who was one of Mr. Ryan’s mentors, was conservative on fiscal policy, like Mr. Ryan. But he was moderate on social policy, which Mr. Ryan is not. With Bill Clinton, Mr. Dole’s opponent that year, performing unusually well for a Democrat in rural areas, Mr. Kemp had the potential to advance Republicans’ standing in the cities.

Mr. Ryan seems to have been chosen specifically because of his conservative policy views and the way he articulates them. Mr. Romney could have done more to push his edge with independent voters by picking Gov. Chris Christie, who has won over moderates in New Jersey. He could have chosen Senator Marco Rubio of Florida, seeking to make headway with Hispanic voters, a critical demographic group.

There were candidates with a longer list of accomplishments, like Gov. Bobby Jindal of Louisiana. There were those with more blue-collar appeal, like former Gov. Tim Pawlenty of Minnesota.

And there were candidates who could have had more impact on the electoral math. Mr. Ryan has some value to the ticket here — he could put Democratic-leaning Wisconsin into play. But he does not have as much potential to shift the balance as someone from a true tossup state with more electoral votes — like Senator Rob Portman of Ohio or Gov. Bob McDonnell of Virginia.

Most of these other contenders also take conservative policy positions. (That is probably a prerequisite for any Republican running mate these days.) But for better or worse, they offer them with more window dressing. Mr. Ryan presents his ideas more nakedly, and his brand is associated first and foremost with his conservatism.

Political theory dictates that this is a dubious choice for Mr. Romney, since Mr. Ryan’s views, especially on Medicare, are not likely to poll well with the average voter.

It’s too early to say how the pick will play out, but the early reviews on Mr. Ryan are lukewarm. State and national polls conducted since the selection have shown, on average, a one-point gain for Mr. Romney in his race against President Obama.

It might be thought that a one-point gain is better than nothing. But vice-presidential announcements in past elections have also produced bounces in the polls, and they have usually been larger, averaging about five percentage points. Mr. Ryan is underperforming so far on that basis.

Likewise, polls conducted on the personal favorability of a new vice-presidential candidate are typically strong at first, with the public initially saying they like the candidate by two to one or three to one. Mr. Ryan’s favorability numbers are only slightly better than break even.

The other problem for Mr. Romney and Mr. Ryan is that polling bounces are, by definition, impermanent and can sometimes represent a high-water mark for the new ticket. As the campaign evolves, and the initial surge of enthusiasm for a candidate fades, a running mate’s unfavorable ratings typically increase faster than his favorable ones do. It is possible that Mr. Ryan could have net negative favorability ratings by Election Day, as Mr. Romney does now.

But Mr. Ryan was not a choice made to win a popularity contest.

My first take on the selection was that Mr. Romney had looked at the polls, concluded that he was losing, and deliberately made a high-risk choice that could shake up the campaign — somewhat as Mr. McCain did with Ms. Palin four years ago.

Reporting since then, however, has suggested that Mr. Romney made the pick despite reluctance from his pollsters and others on his political team. So it looks increasingly as though he made a different sort of gamble — more of a true all-in move.

His bet is that the era of triangulation is over: that Republicans can win elections without having to compromise. Or, perhaps, in an era when so few voters are truly undecided, he thinks a robust turnout from the Republican base could be enough to get him the 270 electoral votes needed to win.

Will it work? Political theory, as I mentioned, argues against it, but Republicans have been moving to the right for a couple of decades now, and it is not clear that they have paid much of a price for it.

Since Mr. Bush’s selection of Mr. Quayle in 1988, Republicans have won three presidential elections and lost three. They have controlled the House in 14 of the 24 years since then, and the Senate in 10 of 24. In other words, each party has still been winning elections about half the time.

With Mr. Ryan on the ticket, this year’s election could serve as something of a referendum on the theory. A split outcome — Mr. Obama winning another term but Republicans holding control of at least one chamber of Congress — is still a strong possibility, and would not provide a clear verdict.

Potentially, however, the addition of a highly visible and controversial member of the House to the Republican ticket will increase the correlation between the Congressional and presidential races, making it more likely that one party will make a clean sweep.

If it is Mr. Obama and the Democrats, Republicans’ success in the midterm elections of 2010 will look like a blip. Democrats would then have won four of the last six presidential elections — and five of six in the popular vote. They would have passed the most significant change to the welfare state in decades, Mr. Obama’s health care law, and paid a relatively small price for it (the loss of one branch of Congress for two years). And they will have done this despite deep voter dissatisfaction with the direction of the economy and the country as a whole.

If Republicans win everything, Mr. Obama’s success in 2008 will look like the outlier — one that could come to be seen as unavoidable given the financial crisis that was unfolding at the time. And the cost of George W. Bush’s controversial war in Iraq? The loss of the House for four years. But by 2014, Republicans would have controlled it for 16 of the last 20 years, and the new Congress would seek to roll back Mr. Obama’s health care law and other accomplishments that liberals made in the interim.

If Republicans are running on Mr. Ryan’s ideas — and winning with them — they will be able to advance a more credible claim that they have a mandate from the public, and that our politics really have shifted to the right.

My view is that the choice of Mr. Ryan will reduce Mr. Romney’s chances of winning the election relative to some of the alternatives. But there is a heck of a payoff if it works.