Barack Obama's decision to support a sweeping expansion of the government's surveillance capabilities - in defiance of his liberal Democratic base - reminds me of the true story about Bill Clinton and the brain-damaged death row convict.

Back in 1992, candidate Clinton, the new kid on the block, was anxious to demonstrate to centrist swing voters that he wasn't a soft-on-crime liberal. He had to show, for example, that he was willing to execute murderers; four years earlier, Michael Dukakis had been defeated in part because he opposed capital punishment. Clinton took care of his problem by terminating Ricky Ray Rector.

In early '92, Rector was living on death row in Arkansas. He had been convicted of killing a cop 11 years earlier. Liberal Democrats generally didn't like the idea of executing the mentally-impaired, but Clinton, who was still governor of Arkansas at the time, ensured that Rector kept his appointment with the executioner. (Rector, who wasn't quite sure what was happening to him, reportedly decided to save part of his last meal "for later.") Anyway, the liberal Democratic base assailed Clinton for his decision, but that was fine with him. He wanted swing voters to see that he had defied the liberals.

Fast forward to Obama. It's the same kind of deal.

Last Friday, Obama announced that he was endorsing the congressional bill that gives President Bush virtually everything he wants with respect to government eavesdropping. Liberals and civil-liberties groups hate this bill for a number of reasons; for starters, it expands the amount of time that the feds can snoop without a court warrant, and it effectively shields the telecommunications companies from the 40 lawsuits that have been filed in the wake of their cooperation with the Bush administration's post-9/11 warrantless wiretapping program.

Obama had pledged last year to "support a filibuster of any bill that includes retroactive immunity for telecommunications companies." But now he's downsizing his vow. He says he's prepared to support the bill as it moves toward Senate passage, pledging only "to work in the Senate to remove this provision so that we can seek full accountability for past offenses."

The idealistic Obama vowed to go to the wall, to block the bill with a filibuster. The calculating Obama says only that he will "work in the Senate," in essence an empty promise. Clearly, he does not want to go to the wall at a time when most Americans, by a margin of 53 to 39 percent, are telling the ABC-Washington Post pollsters that they trust John McCain over Obama to handle the terrorism issue.

The telecommunications provision, by the way, states that AT&T, Verizon, and other defendant firms can squash the 40 invasion-of-privacy lawsuits if they merely prove to a federal judge that they had received written assurances from the Bush team that the warrantless surveillance was legal. In other words, they're off the hook if they got permission slips from the Decider.

The liberal base is ticked that Obama is caving. MoveOn.org, which calls the bill "a get-out-of-jail free card" for the phone companies, wants Obama to honor his original promise. A prominent liberal blogger, Greg Sargent, writes that Obama's new stance is "dispiriting." First Amendment specialist Glenn Greenwald, who blogs frequently on the warrantless surveillance issue, says that Obama's decision is "deeply unprincipled," because, in the candidate's apparent eagerness to meet the GOP's toughness standard, he has opted "to trample upon the political values of those who believe in the Constitution and the rule of law."

But the Obama people clearly are willing to take the hit. It's the classic Clintonian calculation: if you can tick off the base, it reassures swing voters that you're not beholden to the base.

Obama's decision on surveillance also sparks memories of Clinton's "Sister Souljah" moment in '92. That's when Bill publicly assailed the black rapper in front of Jesse Jackson, thereby demonstrating to middle-of-the-road voters that the candidate was not beholden to the Jesse constituency.

Indeed, Obama's decision on surveillance also brings to mind the decision he made last week to spurn public financing and privatize his autumn campaign. (I wrote about that yesterday in a print column, which generated a lot of response.) He angered liberal reformers, who have long sought to defend and improve the public financing of presidential campaigns, and he alarmed some of his own followers, who took note of the fact that he had previously (and repeatedly) signaled his willingness to take public money and live by the spending caps.

But, again, the Obama campaign's core calculation is that liberal voters will stay loyal anyway, even if he persists in using them as a foil. The calculation is that, after eight years of Bush, liberal voters are hungry to elect a Democrat, and they will remain so, even if they're annoyed with the candidate from time to time.

It's really the old political formula: run to the base during the primaries, then run to the center during the general election. Obama's decision to back the Bush-friendly surveillance bill, and thus signal centrist swing voters that he's not an ACLU liberal on the terrorism issue, is right in sync with his first national TV ad, which says nothing about "change" or reform, but instead seeks to reassure swing voters that he bleeds red, white, and blue ("a deep and abiding faith in the country I love").

Obama fans who are overly enamored of the candidate might well heed these two observations: He's a politician. And he wants to win.