If President Josiah Bartlet was the beating heart of The West Wing, then Leo McGarry was its weary conscience. Here was a man of principle, of intellect, of ability, who used his talents in service to his country as a fighter pilot in Vietnam, to amass significant wealth in the private sector, and to drag the best man of his generation to the pinnacle of power. Leo was a fighter and a survivor. Life tested the limits of his boundless capability by shooting down his fighter behind enemy lines, polluting his body with drug and alcohol addiction, and salting his path with enemies of all sorts. He defeated them all, but the key to Leo's character (and his political philosophy) was his conviction that his victories were never his alone, but the product of hands joined together in common effort. Leo was a great man who had no patience for the argument that great men are predators on the earth, taking what they want. Greatness is a function of how much is given, not how much is taken.
Witty, charming, a bit of a clothes horse, Leo could also be grumpy, impatient, and demanding. Hey, he was a busy guy. His face was a map of the world. Leo's relationship with Bartlet was the core of the show and the Bartlet Administration. He was center-left politically: left enough to agree with most of the more-liberal policy goals of his President, but center enough to guide those goals to a more manageable middle. Surrounding himself with the smartest people he could find was another hallmark of Leo's collectivist approach to power, and the constellation of talent he gathered for the campaign and the resulting White House team was startling. Intelligent, ethical, driven staffers supporting a progressive national agenda. There are reasons The West Wing is often regarded as a liberal fantasy.
Whenever I revisit this series (which is every couple of years), I flinch when I get to Leo's two heart attacks. The one that kills him (and, tragically, actor John Spencer) is brutal to experience. But the earlier attack, in the woods at Camp David after his firing by the President, is gut-wrenching, knowing what's to come. Leo deserved better. He deserved the time in the spotlight that would have come with his election as VP with President Santos. He had earned it. Then again, we don't always get the things we earn.
Loyalty was his defining trait, more than any designer suit or loping stride or furrowed brow. Leo had been around, and been in some deep holes, and he knew the secret. Find a friend who has been there before and knows the way out. Leo was that friend to his subordinates, to his President, and to his country.