John Spencer only played a Washington insider on TV, yet “I've been asked to host [FOX News Channel's] The Beltway Boys, I've been asked to debate [Bill] O'Reilly,” a bemused Spencer told PEOPLE in 2003. “I always remind these people that [politics] is not my arena.”

But who can blame them? The actor, who died of a heart attack on Dec. 16, four days before his 59th birthday, played West Wing's shrewd, no-nonsense White House Chief of Staff Leo McGarry to perfection, winning an Emmy in 2002. It helped that the producers drew on Spencer's own hardscrabble life for inspiration: Both Leo and Spencer were divorced; both recovering alcoholics. Still, no one could have anticipated that the coronary the character suffered last season—and survived to run for Vice President—would be followed by Spencer's. “This was a complete shock,” says his friend and publicist Ron Hofmann. Regarding who will replace Leo on the campaign trail, NBC rep Joe Libonati says, “We have episodes shot until March, so there's no need to rush.”

Spencer won't easily be replaced in the hearts of his friends and family. The day he died, he'd been expected to fly from his L.A. home to New York City for his birthday and to prepare for the New Year's Eve party he threw every year. “We had dinner and theater plans. Twenty of his best friends were going to take him out for his birthday,” says his longtime girlfriend actress-choreographer Patti Mariano, 60. “I was wrapping his last Christmas present when my sister called and said, ‘He's in the ICU. Start praying.’”

Mariano had known Spencer since both were students at Manhattan's Professional Children's School in the 1960s. Born John Speshock, the only child of a Totowa, N.J., dump-truck driver and his waitress wife, Spencer left home at 16 to pursue an acting career and soon became a regular on the' 60s sitcom The Patty Duke Show. Though he continued to work steadily in theater and TV, his alcoholism spun out of control in the late 1980s until a cousin finally got him into rehab. When a newly sober Spencer landed a plum supporting role opposite Harrison Ford in 1990's Presumed Innocent, “my life changed overnight,” he said. Soon after he won the part of streetwise attorney Tommy Mullaney on L.A. Law, and he was the first actor cast in The West Wing in'99.

Openly proud of his sobriety (“I wear it like a medal,” he said), Spencer still had an addiction to smoking. “He had quit a year ago, and we were all relieved,” says Hofmann. But early, on Dec. 16, he was admitted to the hospital after having trouble breathing. He died while costar Stockard Channing was visiting him. On Christmas Eve West Wingers Bradley Whitford, Richard Schiff and Nicole Robinson all attended Spencer's funeral. “No matter how difficult it was, people changed their plans and came,” says Mariano. “John was one of the good guys. He was the best.”