“The commitment Mitt Romney and I make to you is this,” Mr. Ryan said, smiling broadly during much of his address. “We won’t duck the tough issues; we will lead. We won’t blame others; we will take responsibility. And we won’t replace our founding principles; we will reapply them.”

In introducing Mr. Ryan, Mr. Romney mistakenly called him “the next president of the United States.” He left the stage, but quickly returned to correct himself, saying, “Every now and then I tend to make a mistake, but I did not make a mistake with this guy.” Mr. Ryan, who had been urged by conservatives to run for president, called Mr. Romney “the right man to lead America back to prosperity and greatness.” He criticized the Obama administration and declared: “We’re in a different, and dangerous, moment. We’re running out of time — and we can’t afford four more years of this.”

The selection of Mr. Ryan means that this campaign is the first in 80 years in which no candidate of either major political party has served in the military. Mr. Romney, a Mormon, and Mr. Ryan, a Catholic, also represent a new era in presidential politics: neither are Protestants.

The two men share an easy rapport and a love of PowerPoint presentations and policy details.

In choosing his running mate, Mr. Romney was looking to elevate a presidential race with a candidate most likely to provide the biggest jolt. It is a gamble, his aides acknowledged, but one they believe is far less risky than Senator John McCain’s selection of Sarah Palin four years ago. Several advisers said that as the author of the party’s central budgetary approach, Mr. Ryan helped make their campaign look “big,” after months of running a race remarkably absent of specific policy proposals.

But in making his choice, Mr. Romney took political ownership of a budget that even some Republicans worry could be a liability in November. Mr. Ryan has proposed sweeping changes in entitlement programs like Medicaid and Medicare, which insure more than 100 million people and whose costs account for more than one-fifth of the federal budget. The selection suggested that Mr. Romney has settled on a strategy of maximizing his support among conservatives rather than trying to win over independent and centrist voters.

While Mr. Romney has pledged to “protect Medicare and Social Security,” his embrace of Mr. Ryan makes it difficult to distance himself from the well-established plans of his running mate. But the campaign suggested it would try to draw a distinction on Medicare, distributing talking points to Republicans that included the question, “Does this mean Mitt Romney is adopting the Paul Ryan plan?” (The answer: He applauds it “for going in the right direction” but will be putting together his own budget.)