We've all heard about artists who suffer for their work. Tony Podesta and his wife, Heather Miller, suffer for other people's. When they bought a 2,000lb Louise Bourgeois sculpture for their home in Washington, for instance, it required substantial renovations to the building. "We had to get a structural engineer in to sort out what sort of support it needed," says Podesta. "And we not only had to build support underneath where it was going but temporary support from the point at which it entered the house to the point at which it was placed. I don't think it'll ever leave."

Then there is the travel involved. The couple have unusually demanding jobs. He is one of Washington's top lobbyists, renowned as the man who first built bridges between Silicon Valley and Capitol Hill. She has joined the lobbying business after a career as a top-flight lawyer in Congress and the Senate. Most people in their positions would spend their leisure time unwinding. Instead, they make what Miller calls "Herculean" trips to Europe and further afield to buy art. It is perfectly normal for them to leave Washington on Friday evening and return the following Monday morning, having visited more than one European capital in the meantime.

Their travelling, and the knowledge of the global art scene they have acquired, has turned them into two of America's best-known collectors. They were meant to be in Rome on vacation when I caught up with them. But the day before they had been out ferreting in Trastevere, where they found a couple of Wolfgang Tillmans photographs, which they unwrapped with engaging enthusiasm.

They are known for purchasing "awkward" works, such as video installations, that many other private collectors will not consider. "It's easy to store them, but difficult to display them," says Podesta. To get round the problem, he and his wife have excavated a huge subterranean vault beneath their house outside Washington - a white space 5m square and 4m high in which it will be possible to show "very complicated video pieces" on all four walls.

Miller, the daughter of academics, still marvels at her involvement in a world and way of life she got to know only after meeting her husband. She recalls one of their first visits to a gallery and how the owner told Podesta he had a work by a particular photographer "going cheap". It turned out he wanted $40,000. "So I'm standing there in front of this photograph and I'm thinking to myself, like, 'This is cheap?' "

Her husband, who looks less like an aesthete than a character from the Sopranos, became involved with collecting no less accidentally. He was helping Ted Kennedy in his failed bid to challenge Jimmy Carter for the 1980 Democratic nomination. When he ran short of cash, Kennedy laid off three-quarters of his staff.

"Those of us who remained were paid in donated art. Robert Rauschenberg, Andy Warhol and Roy Lichtenstein were all supporters," he recalls. "I ended up leaving the campaign with a treasure trove." Collecting became a "form of addiction".

Today, he reckons, he and his wife have the world's biggest collection of Anna Gaskell ("maybe second to Anna Gaskell"). Other contemporary favourites include Gillian Wearing, Marina Abramovic, Sam Taylor-Wood and Olafur Eliasson, whose work Podesta discovered 10 years ago, when the artist was still at the Royal Academy of Arts in Copenhagen.

They are always looking out for little-known artists. Asked to come up with a handful of names that are in the second rank now, but that will one day be in the first, they offer a list that ranges across the world: Britain's Darren Almond, Janaina Tschape from Brazil, Mads Gamdrup from Denmark, the Italian artist Loris Cecchini and Patricia Piccinini, a Podesta protegee who is to represent Australia at the Venice biennial.

It is tempting to see Podesta as a US equivalent of Charles Saatchi - another entrepreneur who has also operated with great success on the margins of politics. Specialising in photographs and video, Podesta is more focused, though he says that his most admired possession is a sculpture, another of Bourgeois's works, "a gloriously beautiful carved marble".

But, for pure investment reasons, did he and his wife ever buy works they didn't like? "We don't buy anything we don't love," he says. "Some people collect one of lots of different people. Our style is to collect a smaller number of artists - probably 50 - in some depth. We'll have 20 of this person and 40 of that person."

His take on the role that he and his wife play is modest. "Frequently, work that we've had in our home has been the first work that museum curators have seen and then decided to put in museum shows. Dealers do not become wealthy in the course of dealing with young artists, so it's a pleasure to support the dealers and the young artists and then help the work find its way into museums."

·Taylor-Wood says

Tony Podesta is someone who has intrigued me for a long time, partly because he is a very passionate and brave collector of art, partly because he's constantly travelling. I wanted to read about him because I admire his spirit - not to mention his resilience to jetlag'