Mitt Romney has selected Rep. Paul Ryan of Wisconsin as his running mate in the presidential race and will make the long-awaited announcement here Saturday morning.

Romney’s campaign told supporters at about 7 a.m. via an iPhone app that the seven-term congressman would be the Republican vice presidential nominee.

Two others who were high on Romney’s short list of VP contenders — former Minnesota governor Tim Pawlenty and Ohio Sen. Rob Portman — were informed in recent days that they would not be on the ticket, according to two Republican sources. Both were scheduled for other events on Saturday.

Among the early clues that Ryan, who chairs the House Budget Committee, was the likeliest pick was the venue chosen for the early morning announcement — the USS Wisconsin in Norfolk. And overnight, the Web site automatically began redirecting readers to the Romney campaign’s home page.

The selection of Ryan would add to the ticket the intellectual architect of the GOP’s approach to slashing deficits and signal a desire to place the nation’s looming fiscal challenges at the center of the campaign’s final months.

Ryan got a strong boost over the past few days from conservatives, with editorials pushing his candidacy in the Wall Street Journal, the Weekly Standard and the National Review.

Romney’s announcement will come at the start of a four-day bus trip through Virginia, North Carolina, Florida and Ohio, and at a time when the former Massachusetts governor has fallen behind President Obama in the polls.

Romney plans to announce his decision about 8:45 a.m., when he tours the USS Wisconsin, a decommissioned World War II-era Navy battleship named to honor the state.

Romney’s campaign advisers reached late Friday refused to discuss what has been a confidential process and noted that Romney had promised to reveal his decision to supporters via a campaign iPhone application.

With some conservatives fretting publicly that he has lost the summer, Romney is under pressure to alter the course of his campaign. He will have several chances to do so over the next three weeks. At the end of the month, Republicans will gather in Tampa for a four-day convention that will provide the former Massachusetts governor a chance to reintroduce himself to the country and reset the race on his terms.

Romney’s effort gets under way in Virginia, where he begins a four-day bus tour through swing states on Saturday with his wife, Ann.

If Romney announces Ryan as his running mate, he would offer voters the starkest possible choice on how to address issues of spending and taxing, embracing Ryan’s single-minded focus on reducing the nation’s debt without raising taxes by dramatically altering Americans’ relationship to their government.

It is a fight Democrats, too, have savored, believing the details of Ryan’s budget will convince voters that they offer a fairer path to reduced deficits through a combination of spending cuts and higher taxes on the wealthy.

Ryan would likely energize a GOP base that sought a campaign of forward-looking ideas and not just an endless critique of President Obama.

Romney’s bus tour is aimed at showing the candidate connecting with blue-collar America. It is designed to help Romney shed the caricature that the Obama campaign has tried to draw of him as an elitist who looks out only for the wealthiest. In addition to touring the USS Wisconsin, Romney plans to swing by a bakery in Ashland, Va., on Saturday. He will stage a rally Sunday at the NASCAR Technical Institute in Mooresville, N.C., near Charlotte.

Aboard his campaign plane Friday evening, Romney told reporters: “Bus tour, it’s great! It’s great to be out campaigning. . . . Campaigning is the most fun, is the most enjoyable and rewarding.” Romney then clapped his hands and returned to his seat at the front of the plane. “Back to my yogurt,” he said.

Romney’s focus on the middle class will carry into the convention at the end of this month. The national spotlight will shine brighter in Tampa than at any other moment in Romney’s campaign, and his supporters are betting that many voters will form their impressions of him then.

“To borrow a phrase, the convention has the potential to be an ‘Etch a Sketch moment,’ ” said Mark McKinnon, a longtime Republican image-maker who advised the presidential campaigns of George W. Bush and Sen. John McCain. “Conventions often wipe the slate clean. A crucial bloc of undecided voters, though there aren’t many this year, will just start paying attention when the convention starts.”

Romney’s advisers are putting the finishing touches on plans to reintroduce the candidate to the nation. They have filmed new videos with Romney and his family at his lakefront vacation home in Wolfeboro, N.H., and are devising a prominent role onstage for Ann Romney and their extended family.

“Americans are going to get a real close look at Governor Romney, his wife, Ann, and the entire family,” senior adviser Eric Fehrnstrom told reporters here Friday. “I think that they’re going to be impressed by the fact that this is a family that shares their values. He shares their values of hard work, of patriotism, of sacrificing so that the next generation has it better than the current one.”

For Romney, a big moment couldn’t come soon enough. Weeks of pummeling by Democratic ads depicting Romney as an out-of-touch plutocrat and possible tax evader appear to have taken a toll.

Three national polls released over the past two days show Obama widening his lead over Romney to as much as nine points. The surveys of registered voters, all conducted between Aug. 2 and Aug. 8, also show Romney’s unfavorable ratings rising. Two of the polls show his support among independents slipping.

A Fox News poll found the largest deficit, with Romney trailing by nine points — 49 percent to 40 percent — the widest gap Fox has reported all year.

A senior Romney adviser played down the new polls at a news briefing Friday morning at Boston headquarters, saying that they must be midsummer flukes because there had been no “precipitating event” to move the numbers so much.

The adviser pointed to the latest Gallup tracking polls, which have the two candidates in a dead heat, as well as to Rasmussen, an automated poll that usually leans Republican and shows Romney ahead of Obama.

Bill Turque and Rosalind S. Helderman in Washington contributed to this report.