The new senator from Minnesota is a comedian, writer and actor who lived on the Upper West Side of Manhattan and raised a lot of money from friends in Hollywood. The departing governor of Alaska is a hockey mom from a small backwoods town who likes to hunt and fish. Yet today, Al Franken looks wholesomely mainstream, while Sarah Palin seems headed for the tabloid fringe.

That unexpected contrast reveals much about the current configuration of Republicans, Democrats and politics in America — a story of two parties that crossed paths while traveling in opposite directions over the past dozen years or so.

Before he entered politics, Franken had a long and highly successful career in television, wielding a sense of humor that could be wicked, outrageous and even offensive. He was an urban denizen with liberal sensibilities who counted professional wrestlers, college professors, scruffy journalists (including this one) and members of the Grateful Dead among his friends. Even after he signed on as the star host of the progressive Air America radio network, he was primarily an entertainer.

Back around the time that Franken quit “Saturday Night Live” for the second time, Sarah Palin entered public life as a civic activist and candidate for local office in Wasilla, Alaska, where she was soon elected mayor. She was a populist of the right-wing variety, a fundamentalist churchgoer and a scourge of politics-as-usual. Concerning herself with such conservative staples as government spending, tax cuts, term limits and gun rights, she was a textbook Republican officeholder.

But somewhere along the line, everything changed for both them and their parties. Franken left showbiz behind to prove himself a serious policy wonk as well as a devoted family man; Palin transformed herself and her family into a reality television show.

The entertainer became a public servant — and the public servant became entertainment.

How this all came to pass is a complicated story that actually begins long before the political decisions that led to his rise and her fall. The bookish, wise-guy Al always had a political streak dating back to his college years at Harvard, where he switched from mathematics to political science and graduated with honors. The telegenic, athletic “Sarah Barracuda” embarked on a career as a TV sportscaster in Alaska’s biggest city before eloping with Todd and moving home to Wasilla.

The reversal of the parties’ trajectories, in style and substance, may have begun during the 2002 election, a stunning midterm defeat for the Democratic opposition that Republican strategist Karl Rove predicted to be the start of decades of unchallenged rule for GOP conservatism. That was also the moment when Paul Wellstone, the Democratic senator from Minnesota who had become a national icon of progressive politics, died in a terrible plane crash along with his wife, Sheila, his daughter Marcia and three aides, as he was campaigning for re-election.

In the bitter aftermath of his death, turncoat Democrat Norm Coleman won a special election to succeed Wellstone, and joined the Republican majority in the Senate. And Franken, a Minnesota native, began to think about whether he might someday run for that same seat to vindicate the legacy of Wellstone, one of his closest friends.

In victory, the Republicans grew increasingly extreme and overconfident, encouraging figures such as Palin to follow their most extreme instincts. In defeat, the Democrats at last began to refurbish their progressive ideology, reconnect with working American families and rediscover their will to fight.

As an author and radio personality, Franken made a significant contribution to his party’s renewal. He was ready for prime-time politics in ways that Palin, the sudden star who could barely utter a coherent paragraph, was not.

Beneath the glittering surface, she exhibited profound weakness. Behind the joking persona, he showed moral and intellectual strength.

Joe Conason writes for The New York Observer.