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

Romney’s campaign formally alerted supporters at 7 a.m. via an iPhone app that the seven-term congressman would be the Republican vice-presidential nominee. Minutes later, the campaign sent out a release calling Romney and Ryan “America’s comeback team.”

By selecting Ryan, Romney has made a potentially bold but risky move to reset the dynamics of the presidential election. He has chosen the intellectual heart of the movement within the Republican Party to slash deficits and signaled a desire to place the nation’s looming fiscal challenges at the center of the campaign’s final months.

Romney will formally introduce Ryan as his new partner at a rally in the key swing state of Virginia — appearing with the congressman on the USS Wisconsin in Norfolk, a not-so-subtle nod to the pick.

Already, the campaign has unveiled a new Web site, www.RomneyRyan.com, redirecting readers to the Romney campaign’s home page.

With his selection, Romney offers 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.

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

Ryan’s plan to overhaul Medicare is particularly controversial and will now play a key role in critical swing states with large retiree populations, most notably Florida, where Romney and Ryan are scheduled to campaign on Monday.