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A long list drawn up in April of possible contenders was narrowed by May. Mr. Romney called the people on it to ask if they were willing to undergo the intensive vetting process.

All said yes. Everyone who was asked submitted detailed questionnaires and paperwork, including several years’ worth of tax returns.

The volunteer lawyers sequestered in a secured room pored over the documents. They dug through public records. Each night, they locked the paper in safes in the Boston headquarters.

Some questions came up. So in June, during an event for donors in Park City, Utah, Ms. Myers met with several candidates to clarify a few more details with each. On July 2, she met with Mr. Romney at his home in Wolfeboro, N.H., where they went through folders on each person under consideration.

On Aug. 1, the day after returning from a weeklong trip abroad, Mr. Romney met one last time with the nine advisers he had consulted throughout the process, including longtime friend Bob White, campaign manager Matt Rhoades and years-long confidante Ron Kaufman.

It was one final gut check.

That afternoon, Mr. Romney called Mr. Ryan from Ms. Myers‘ office. Would he come to Boston to meet with Mr. Romney?

Yes, came the reply.

So Mr. Ryan — asked to dress casually — arrived at O’Hare Airport in Chicago in jeans, a baseball cap and a pair of sunglasses. No one recognized him. He flew to Hartford, Conn., where Ms. Myers‘ 19-year-old son, Curt Myers, picked him up in a rented SUV and drove him to the Myers family home in Brookline, Mass. Meanwhile, a Secret Service detail drove Mr. Romney from his lake house in New Hampshire, and the two men met alone for over an hour in Ms. Myers‘ dining room.

“We talked about the campaign and how it would be run and talked about how we’d work together if we got the White House; what the relationship would be, how we’d interact, and be involved in important decisions,” Mr. Romney said of the meeting as his campaign plane flew from Virginia to North Carolina late Saturday.

Mr. Romney and his team had been looking for someone who could play a partnership role in the White House.

Aides involved in the selection had read Robert Caro’s biography of Lyndon Johnson as a cautionary tale: As John F. Kennedy’s vice president, Mr. Johnson was often cut out of major policy decisions.

When they walked out of the dining room, Mr. Ryan had agreed to be on the ticket.

“By the time we met in person, I kinda knew it was gonna happen,” Mr. Ryan said late Saturday. “And, ah, I was very humbled. It was the biggest honor I’ve ever been given in my life.”

While Mr. Ryan was still at Ms. Myers‘ home, news broke of the shooting that killed 6 at a Sikh temple in Wisconsin.

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