In making an endorsement before his state's primary last spring, Ryan said, "I picked who I think is going to be the next president of the United States — I picked Mitt Romney. ... The moment is here. The country can be saved. It is not too late to get America back on the right track. ... It is not too late to save the American idea."
Romney was the subject of an April Fools prank in which Ryan played a role. Romney showed up at a supposed campaign event where he heard Ryan calling him "the next president of the United States" — only to find the room nearly empty.
In recent days, conservative pundits have been urging Romney to choose Ryan in large part because of his authorship of a House-backed budget plan that seeks to curb overall spending on benefit programs such as Medicare, Medicaid and food stamps.
Republican National Committee finance chairman Ron Weiser of Michigan, said Friday night that Ryan's selection would help Romney win Wisconsin and its 10 electoral votes in the fall. The state typically supports Democrats in presidential contests, and Obama won it handily four years ago.
Ryan has worked in Washington for much of his adult life, a contrast to Romney, who frequently emphasizes his experience in business.
The congressman worked as an aide in Congress, and also was a speechwriter for Jack Kemp, who years earlier had been one of the driving forces behind across-the-board tax cuts that were at the heart of Ronald Reagan's winning presidential campaign in 1980.
Ryan is also well-known for his fiendish physical fitness workouts.
His congressional district in southeast Wisconsin has something of a bipartisan voting record. Obama took 54 percent of the vote there in 2008, while the congressman received 64 percent in winning re-election.
Outside Ryan's home in Janesville, Wis., on Friday night, there was nothing to suggest that the residence belonged to a vice presidential candidate. An Associated Press reporter who knocked just before midnight got no answer. There was a light on in a first-floor room of the two-story brick home atop a hill.
Earlier this week, a Ryan adviser said the congressman, his wife and their three children were preparing for a weeklong Colorado vacation.
Most of Romney's staff learned of the planned announcement during a 10 p.m. EDT conference call Friday about an hour before the campaign issued a statement. The identity of Romney's pick was not disclosed during the call. The campaign had promised that first news of the selection would be delivered via a phone app.
Earlier in the day, Romney's campaign briefed reporters on the bus tour without mention of the impending vice presidential announcement.
The tour will take Romney through North Carolina, Virginia, Florida and Ohio. All are battlegrounds where Obama won in 2008. They hold 75 electoral votes combined, of the 270 needed to win the election.
AP reporters Steve Peoples and Andrew Taylor in Washington and Todd Richmond in Janesville, Wis., contributed to this report.