Alternate headline: “Lone popular Republican in extremely liberal state not seizing opportunity for some reason.”

In fairness, he’s had to push the electoral boulder up Massachusetts’s blue mountain twice in three years, the last time with the House GOP’s unpopularity tied to his ankle. The guy must be exhausted.

Scott P. Brown said on Friday that he had opted out of the Senate race in Massachusetts to fill the seat being vacated by John Kerry…

“I have received a lot of encouragement from friends and supporters to become a candidate, and my competitive instincts were leading in the same direction,” he said in a statement.

“Even so, I was not at all certain that a third Senate campaign in less than four years, and the prospect of returning to a Congress even more partisan than the one I left, was really the best way for me to continue in public service at this time,” he said. “And I know it’s not the only way for me to advance the ideals and causes that matter most to me.”

If you’ve read any analyses of Brown’s options lately, and there have been plenty over the last few weeks that were skeptical he’d run again, you already know the factors that were weighing on him. One: For whatever strange reason, it’s a lot easier to get elected governor in Massachusetts as a Republican than senator. Until Deval Patrick won in 2006, Mass hadn’t had a Democrat at the top since 1991. Whereas, until Brown won in 2009, they hadn’t elected a Republican to the Senate since Edward Brooke in 1973. Patrick isn’t running again, so Brown can run for governor in 2014 and not have to worry about an incumbent. (He could also run for senate again in 2014, actually, when Kerry’s term ends. But why do that if you’re not willing to run now in the special election and try to regain the advantage of incumbency?) And if he’s thinking of running on a national ticket someday, be it GOP or third party — are you ready for Christie/Brown? — executive experience would be useful.

Two: After facing him in two elections, one with a disastrous candidate and one with a less disastrous one, Democrats will be ready for a third match with Brown. Quote:

“The atmosphere would be completely different,” said the state Democratic Party chairman, John Walsh. He acknowledged making “unforgivable mistakes” by taking for granted the race in which Brown won the late Sen. Edward M. Kennedy’s seat.

“We are not asleep at the switch anymore,” Walsh said.

For all his charm and personal popularity, notwithstanding Elizabeth Warren’s underwhelming campaign and poor retail skills, Brown lost by seven and a half points last year. Ed Markey, the likely Democratic nominee, is underwhelming too, but Brown would have extra baggage this year in seeking to rejoin a GOP caucus that’ll be locked in death struggles with Obama over guns, taxes, and immigration for the rest of the spring and summer. How many times could the guy realistically be expected to defy political gravity in Massachusetts?