“I’m not a real believer in limits,” Mr. Johnson said.

Mr. Johnson rarely speaks at length with reporters. But in a series of conversations, he said he was motivated by a belief in Mr. McCain and the democratic process.

“I only take on things I really believe in,” he said.

But Mr. Johnson also clearly has his own agenda. Staff members on Capitol Hill credit him with playing a pivotal role in 2002 in pushing members of Congress, including Speaker J. Dennis Hastert, to allocate $750 million over five years for juvenile diabetes research. Mr. Johnson’s oldest daughter, Casey, has Type 1 diabetes, and he has given millions to the search for a cure.

“We sat down and talked a couple times,” said Mr. Hastert, who added that he and Mr. Johnson had bonded over football. “He made a very good case that by investing U.S. dollars, we could actually save money.”

Mr. Johnson, who has another daughter with the autoimmune disease lupus and raised millions for that cause, also met with President Bush in the White House to push for embryonic stem-cell research, a meeting Mr. Johnson believes might have helped Mr. Bush to compromise in his policy and still allow federal financing for research on existing stem-cell lines.

Mr. Johnson’s political clout has certainly not hurt him as owner of the Jets and in the search for a new stadium for the team, even if his quest to build one in Manhattan ultimately fell short. He is candid about the need to make contributions to New York and New Jersey Democrats as well, given his business interests in the region.

The Jets and the Giants are building an $1.6 billion stadium together in New Jersey, but some critics have questioned the wisdom of the state’s assuming $100 million in debt to pay off the teams’ old stadium as part of the deal.

Like other major donors, Mr. Johnson has traveled with Mr. McCain on the campaign trail. Mr. McCain also calls him on occasion to thank him. But Mr. Johnson plays down the access he has, saying he is no different from anyone else.