When Sen. Olympia Snowe announced her retirement, it recalled a day almost exactly 18 years earlier, when Sen. George Mitchell made a similarly late announcement in a re-election race everyone expected him to make.

After all, Mitchell was a much-admired Senate majority leader, the only Mainer to lead either house of Congress since James G. Blaine in the late 19th century, and he’d already turned down President Clinton’s offer to appoint him to the Supreme Court.

Snowe’s decision was described as “shocking” and “astounding,” but wasn’t nearly as out-of-the-blue as Mitchell’s. At age 60, Mitchell had been in the Senate for 14 years – Snowe has been in Congress for 34 – and he hardly seemed ripe for retirement. Since he went on to broker a peace agreement in Northern Ireland, run the Disney Corp., help clear up Major League Baseball’s drug scandal and serve as President Obama’s Middle East envoy, it could be said that his post-Senate career exceeded his Washington accomplishments.

Snowe’s situation is different. At age 65, she’s spent her entire adult life in the Legislature and Congress. She herself named the biggest problem – the intense polarization of Washington, and the disdain for compromise-makers, the role Snowe preferred.

Unlike Mitchell’s clear path to re-election, Snowe’s would have been stormy, though she’d likely have prevailed. Her Republican primary opponent – her first since 1978 – had already demanded her resignation over alleged improprieties in the college loan company where her husband, former Gov. John McKernan, was CEO, and on which their substantial fortune rests. Four Democrats were also running; the race would have been long and demanding.

And Snowe clearly wasn’t looking forward to six more years in Washington. We often see elected leaders as solely political animals, but they’re not. They have the same human needs and failings as the rest of us, and it’s quite possible to have had enough of Washington. Mitchell said he left to spend more time with his wife and two young sons, and doubtless believed it.

In retrospect, Snowe’s departure began taking shape in late 2009, when she voted on the Senate Finance Committee in favor of the Affordable Care Act (ACA), which she was hoping to convince other Republicans to support. None did, and she felt compelled to vote against health reform on the floor. Voting on the most important domestic legislation in decades fell entirely along partisan lines – an unfortunate result for an important national undertaking.

Snowe found herself in the same position now bedeviling Mitt Romney, who’s repudiated his own major legislative achievement as governor, a health reform law that’s given Massachusetts the nation’s lowest uninsured rate — a law remarkably similar to the ACA.

Snowe had good ideas. She proposed that a public option to compete with private insurance be added with a “trigger” to activate it if for-profit insurers didn’t moderate rate increases, which they haven’t. Had her advice been heeded, the ACA would now be in better shape. It remains vulnerable to manipulation by for-profit insurers, who stand to gain 30 million new paying customers.

Snowe is the last moderate Senate Republican. Even her Maine colleague, Susan Colllins, is more conservative, and that cannot be a good thing. If polarization is a big problem – and it is – it’s in large part because the GOP has moved further and further right. There’s little in Obama’s Democratic Party that wouldn’t be embraced by Clinton or even Jimmy Carter.

The jockeying to replace Snowe is already in full force, with the early edge to the Democrats. 1^st District Rep. Chellie Pingree has the inside track now that her 2^nd District counterpart, Mike Michaud, has bowed out. Eliot Cutler or former Gov. Angus King might run as independents. King could be formidable, but he’s also 68 and left politics 10 years ago. Any Republican entering the race, and many are looking, will lack the cachet with voters Pingree already enjoys.

Indeed, it was her seven terms in the House that gave Snowe an edge when she ran for, and won, Mitchell’s seat, which had been occupied by Ed Muskie for 22 years before that.

Bill Cohen, Snowe’s mentor, who himself unexpectedly left the Senate two years after Mitchell, once told me that small states like Maine benefit from having a Democrat and a Republican in the Senate. There’s always someone to bring influence to bear, whichever party holds power. So a Sen. Pingree or Michaud, paired with Collins, could benefit Maine as did the durable Mitchell-Cohen combo, or Muskie and Margaret Chase Smith before that.

There’s campaigning, and then there’s governing. They’re definitely not the same.