Eric Thayer for The New York Times

Mitt Romney is scheduled to announce his vice-presidential candidate on Saturday in Norfolk, Va., with several signs pointing toward Representative Paul D. Ryan of Wisconsin emerging as the leading candidate for the position.

Mr. Romney is set to disclose the selection as he tours the battleship U.S.S. Wisconsin at 8:45 a.m. on Saturday, the campaign announced Friday evening. The announcement ends a four-month search for a running mate on the opening day of a four-day bus tour through four critical battleground states.

A confidant of Mr. Ryan’s confirmed early Saturday that aides believed Mr. Romney had settled on the Wisconsin congressman to join the Republican ticket, but all advisers had been sworn to secrecy. Three senior Republican officials said that they, too, believed that Mr. Ryan, the chairman of the House Budget Committee, had emerged as the top choice.

Later on Saturday morning, the Web address romneyryan.com began pointing to Mr. Romney’s campaign Web site.

Mr. Romney telephoned other finalists for the position on Friday evening, a senior Republican aide said, and thanked them for their cooperation in the vetting process and their help with his campaign effort. But he did not tell them whom he had decided upon.

Tim Pawlenty, the former governor of Minnesota, was set to campaign for Mr. Romney in New Hampshire on Saturday. A Republican close to Mr. Pawlenty said that he had been informed that he had not been chosen by Mr. Romney; Mr. Pawlenty would continue with his schedule and not be in Virginia for the vice-presidential announcement, the Republican said.

Senator Rob Portman of Ohio, who had also been vetted and considered a top prospect, received a call from Mr. Romney on Friday night informing him that he had not been selected, a Republican official confirmed. Mr. Portman is scheduled to attend a cancer research bike ride in Ohio on Saturday and was not asked to travel to Virginia for the announcement.

Two top advisers to Mr. Romney declined to provide more details late Friday, saying that Mr. Romney valued the secretive process that has been carried out throughout the four-month search for a running mate. Mr. Romney and his choice are scheduled to travel to North Carolina, Florida and Ohio in the coming days on an introduction tour leading up to the Republican convention at the end of the month in Florida.

For months, Mr. Romney and a close-knit group of advisers have quietly vetted potential candidates, scouting their records, personal lives and public appearances in a process that has produced remarkably few unwanted leaks, even at this late date.

Aides to Mr. Romney had long promised that they sought to cleverly divert attention from the timing of their vice-presidential announcement — and they did just that on Friday. The day began with a briefing at the campaign’s headquarters in Boston, during which the adviser in charge of the search process, Beth Myers, told reporters that she was looking forward to dropping her daughter off at graduate school this weekend. On Mr. Romney’s plane Friday afternoon, which was repeatedly delayed by thunderstorms, he appeared relaxed and playful in jeans, holding court in the center aisle with aides. But there were a few telling signs: many of Mr. Romney’s most senior advisers, who rarely travel together, joined him on the ride to Virginia from Boston, for example.

Even early Saturday morning, the Romney campaign sought to keep details of the announcement cloaked from view, asking police officers to remove reporters who had flocked to the docks surrounding the U.S.S. Wisconsin to glimpse preparations for the event.

Should Mr. Romney choose Mr. Ryan, one of the leading figures of the fiscal conservative movement, the pair would present a Republican ticket that offers a sharp distinction for voters to decide which party has the better answer to tackle the country’s deep financial burdens.

Eric Thayer for The New York Times

Mr. Ryan, 42, is a rising star in the Republican Party and a favorite among conservative activists who view him as deeply committed to their fiscal principles.

But Mr. Ryan, a member of Congress since 1999, is also a lightning rod for Democrats who view him as the driving force behind Republican efforts to sharply cut social spending and entitlement programs.

He emerged with those dueling reputations over the last several years, and became a more public symbol soon after Republicans seized control of the House of Representatives in the 2010 midterm elections.

As chairman of the House Budget Committee, he pushed his colleagues to boldly stake out an uncompromising position on budget matters, sometimes making his colleagues uncomfortable about the political dangers of his positions.

Those dangers were voiced by former House Speaker Newt Gingrich last year, who called Ryan’s budget plan “right-wing social engineering” during an interview. He was quickly and roundly scolded by his Republican colleagues.

Mr. Ryan has emerged as one of the Republican Party’s ideological leaders, a fiscal conservative intent on not repeating what he believes were deep spending mistakes during the George W. Bush administration. The Ryan alternative to the Obama administration’s budget — once seen by many Republicans as too politically fraught, with its blunt talk on Medicare and Social Security — has become the core of the Republican Party’s fiscal plan.

He was a central pillar in winning the Congressional majority in 2010, persuaded his party to embrace a “Roadmap for America’s Future,” and promoted himself as one of the party’s new leaders who called themselves the Young Guns. He has recommended making significant changes to Medicare, the popular health insurance for older Americans, and Medicaid, which provides health coverage for the poor, and privatizing Social Security.

The ideas initially made him all but radioactive, even among Republican leaders, but he steadily built support for his plan among conservative commentators and the party’s leaders and eventually persuaded his colleagues in Congress, as well as the party’s presidential candidates, to join him last year.