(Image Credit: Don Emmert/AFP/Getty Images)

BOSTON - Mitt Romney today revealed for the first time that he has put longtime adviser Beth Myers in charge of his vice presidential vetting process.

"I have selected someone who has been a counselor of mine for a number of years, Beth Myers. She was my chief of staff when I was governor," the former Massachusetts governor told ABC News' Diane Sawyer in an exclusive interview.

"I've asked her to be the person who oversees the process of the vice presidential selection and vetting an analysis and so she's begun that process and is putting together the kinds of things you need to do to vet potential candidates," Romney added.

See the exclusive interview today at 6:30 p.m. ET on "World News With Diane Sawyer" and 11:35 p.m. ET on "Nightline."

Asked whether there is a deadline for choosing his running mate, the presumptive GOP presidential nominee said he does have a deadline in mind but would not reveal the specifics.

"It would certainly be by the time of the convention," he said. "I don't think we've chosen the time we'd actually make an announcement," Romney said during a stroll through Fenway Park, where he is scheduled to join two contest winners at a Red Sox game.

Ann Romney said this weekend, which the couple spent in Florida fundraising, was the first time they had actually spoken seriously about a vice presidential pick.

"You know it's been interesting this weekend was the first time we seriously really talked about it and there are some wonderful people out there," she told Sawyer. "So we're thinking about it now and we haven't allowed ourselves to go there yet and I think this is a time when we realize it's very important and it's time to think about it."

"There are some pretty terrific people out there," she added.

Romney, who recently spoke about the need to appeal to Hispanic voters, said  Florida Sen. Marco Rubio is "one of the terrific leaders" of the Republican Party, but wouldn't go as far as to say whether he is on the shortlist.

"I think it's way too early to begin narrowing down who the potential vice presidential nominees might be," Romney said. "But we're beginning that process."