The way the GOP primaries are going, Barack Obama may not have to campaign for re-election. The Republican presidential candidates are doing it for him.

Republicans approached the start of the primary and caucus season with a field of second-string candidates, except for Mitt Romney and arguably Newt Gingrich, once of Cobb County. For widely varying reasons, the GOP’s political stars declined to enter the race, even though Obama appeared clearly vulnerable.

Even with the junior varsity, two candidates, Herman Cain of Atlanta and Gary Johnson, never made it as far as the season opener, the Iowa caucus. Michelle Bachmann dropped out soon after.

Saturday’s South Carolina primary is looking less and less like the make-or-break event it once was. The process leading up to it was enlivened by a series of debates that were less a clash of ideas than a sort of verbal game of Whac-A-Mole.

This past week, Jon Huntsman, who had strong credentials in the field in terms of foreign policy experience, dropped out after an indifferent showing in New Hampshire. He departed the scene with an offhand endorsement of Romney that was almost an afterthought.

On Thursday, Texas Gov. Rick Perry, from whom political handicappers expected great things, also dropped out, his poll numbers having tanked after a series of gaffes. In what seems to be becoming a tradition in this race, he left the stage with a backhanded endorsement of a competitor: “Newt Gingrich is not perfect, but who among us is?” There’s a great bumper sticker.

That leaves: Gingrich with his checkered past; three wives, two of whom he cheated on, as the public was reminded of on Thursday via a “spill-the-beans” interview with ex-wife No. 2, and his huge ego. Rick Santorum, whose views do not play well outside the hard-right wing of the Republican Party and whose personality becomes wearing on repeated exposure. And of course Ron Paul, who seems to be campaigning in a parallel political universe, although admittedly he does very well there.

It also leaves Romney, a hugely successful businessman, former governor and the consensus frontrunner.

Obama’s biggest political weakness is the economy, particularly joblessness. Romney is best poised to exploit that weakness by pledging to energize the private sector, but his fellow Republicans seem to be doing their best to see that he doesn’t.

Perry stigmatized him as a “vulture capitalist” and Gingrich was quick to pile on. Romney has been beaten about the head for the excesses of venture capitalism, his low but legal tax rate on investment income and his business bank accounts in the Cayman Islands.

In the process all of the hopefuls, with the exception of Romney, have heaped trash on Ronald Reagan’s famous dictum: “Though shalt not speak ill of a fellow Republican.”

Sarah Palin says she hopes Gingrich wins Saturday because it will prolong the Republican primary. The Obama White House no doubt hopes so, too.