The New York Times quoted an unnamed senior Republican close to the selection process as saying the campaign intentionally chose to have Romney announce his pick during a morning tour of the battleship U.S.S. Wisconsin.
Ryan, the Wisconsin congressman, is chairman of the House Budget Committee and a rising star in the party.
CNN reported that Romney had begun calling runners-up in the "veepstakes" to tell them of his decision.
The former Massachusetts governor's choice comes as he tries to repair an image damaged by negative Democratic advertising and shift the trajectory of a campaign that has seen him lose ground to President Obama in the latest polls.
The race remains close in those surveys, and the GOP challenger has opportunities to grab public attention, starting with his battleground-state bus tour. The running-mate announcement comes just 16 days before the TV hoopla of a national convention, where Romney will accept the nomination he has sought for nearly a decade.
If the last few days are any indication, Romney will spend the coming weeks trying to gain ground largely by dragging Obama down on a host of issues. It's a shift away from the all-economy, all-the-time strategy of the last several months.
"Doesn't America deserve better than a president who will say or do anything to stay in power?" asks Romney's latest ad that seeks to undercut Obama's character. Aides say it will be on TV, but they won't say where.
That spot - and another running in battleground states that accuses Obama of moving to "gut welfare reform" by dropping work requirements - is part of an effort to use Romney's huge cash stockpile to undermine his Democratic rival. In much the same way, Obama and his allies have ripped into Romney over the last several months.
Three months before the election, the Democrats' latest strategy in appearances and in ads is to assail Romney on taxes - criticizing both his cuts-for-all policy stance and his refusal to make public his personal returns. They say polls show that line of attack is effective. It also allows Obama to talk about the economy without focusing on unemployment.
Of the nearly $500 million spent on television advertising in the campaign so far, Obama's team alone has spent $205 million for spots - most of them negative - compared with Romney's $60 million, according to media trackers. But pro-Romney outside groups have spent big on his behalf, giving Romney the ability to save his own money for the campaign's final stretch when he can control his own message in ads.
Combined, Romney and supportive outside groups, which are hammering Obama at every turn, have spent more than the president and his Democratic allies, $255 million to nearly $230 million. Romney and the GOP also significantly out-raised the president and the Democrats for three straight months. The campaign says Romney, the Republican National Committee, and state GOP parties have nearly $186 million in cash on hand.
Despite those GOP advantages, the Democratic criticism has taken a toll.
Where the race was long virtually tied, polls now show Romney slightly trailing Obama nationally and in key states after a difficult summer that included a misstep-filled overseas trip. Romney aides dismiss those polls as inconsequential and point to voters who are on summer vacation or focused on the Olympics.
Yet, two national polls found the Republican challenger's favorability ratings slipping from a positive tilt to an even divide, while two others found his unfavorable rating has grown 6 points among registered voters since the spring.
Romney's schedule in the coming weeks offers natural opportunities to try to change that.
The vice presidential selection, expected any day, will dominate headlines, and Romney's team has been relentlessly teasing the announcement.
"I can't wait to find out who Mitt will choose as his running mate. I'm just as excited as everyone else to meet the other half of America's Comeback Team!" Romney's wife, Ann, wrote in an e-mail this week, encouraging supporters to visit the campaign website to donate and register for the chance to meet Romney and his pick in person. The campaign has also created a smartphone app designed to gather information from supporters while promising to notify them of Romney's decision as soon as it's made.
The bus tour will include Romney appearances with Sen. Portman, as well as with two others talked about as possible contenders: Virginia Gov. Bob McDonnell and Sen. Marco Rubio of Florida.
The tour starts Saturday and will take Romney through four must-win states in as many days: North Carolina, Virginia, Florida, and Ohio. All are battlegrounds where Obama won in 2008. And while the president could afford to lose in one or more of them and still reach the 270 electoral votes needed for another term, Romney almost certainly needs all four to beat him.
Inquirer political writer Thomas Fitzgerald and Matt Katz contributed to this article.