WASHINGTON — It isn’t easy being a caucus of one.

Sometimes you don’t even agree with yourself.

Just last month, South Carolina Republican Sen. Lindsey Graham — the Senate Democrats’ indispensable man on immigration reform — was insisting that President Obama “step it up” on immigration if he hoped to see results.

Just last weekend, Graham — the Senate Democrats’ indispensable man on climate change — pulled the plug on an about-to-be unveiled bipartisan energy bill because, he said, there was too much focus on immigration. Graham denounced the “hurried, panicked” shift to immigration as “a cynical political ploy.” By Monday night, he was demanding assurances from Senate Majority Leader Harry Reid that immigration be tabled for the year.

Perhaps not coincidentally, Reid is in a tough re-election race in a state with a large, and so far largely unmotivated, Hispanic population.

You could understand why Reid would be vowing to move on immigration reform, even if no package is close to ready — and even if he can’t count 50 votes for reform, much less 60. You could also understand why, having barely gotten health reform through, he might not be anxious to bring up an energy bill certain to be assailed as another governmental intrusion into the economy.

Perhaps not coincidentally, Graham’s closest friend in the Senate, John McCain of Arizona, is in a tough re-election race in a state with a large number of illegal immigrants, a noxious new immigration law — and a primary opponent flaying McCain for his previous squishiness on the topic.

You could understand why Graham might want to spare recovering maverick McCain from an immigration debate. And you could understand why Graham, censured by three chapters of the South Carolina Republican Party for fraternizing with Democrats, might want to ease some of the heat he’s been taking for pushing “Grahamnesty.”

Graham — until recently, anyway — has reveled in his role as the Republican Man to See: breaking ranks to be the only Republican on the Senate Judiciary Committee to back Supreme Court nominee Sonia Sotomayor; negotiating with John Kerry and Joe Lieberman on climate change; dickering with Chuck Schumer on immigration; horse-trading with Rahm Emanuel on closing Guantanamo and trying terrorist detainees.

So far Graham has managed to carve out political space for this admirable unorthodoxy, sacrificing some support among his state’s Republican base but making it up by attracting independent and Democratic voters.

The disturbing question is whether this remains a safe path in a tea-party era when political line-crossers such as Florida Gov. Charlie Crist are endangered and even stalwart conservatives such as Utah Sen. Robert Bennett have to worry about primary challenges from the right.

In South Carolina, the attacks on Graham have gotten increasingly ugly. Graham “has shown incredible courage,” a senior administration official told me before the recent eruption. “He has willingly taken on party orthodoxy at his own political peril. Thank God he’s not up until 2014.”

Given the risks he’s taken and the time he’s put in on climate change, Graham’s frustration is understandable. His political analysis — that the new focus on immigration reform has more to do with electoral politics than any realistic hope of getting legislation passed — is spot on.

Immigration reform is hard even with extensive groundwork; this is why the president — at Graham’s behest — has been phoning around in an effort to find a second Republican willing to join the cause. Forcing senators to take a pre-election stunt vote risks hardening positions for the future. Climate change is hardly easier but the moment, at least, is riper, with an unlikely array of backers ranging from business to the Christian Coalition to environmental groups.

Nonetheless, there was something outsized about Graham’s dramatic public pullout. Immigration wasn’t about to leapfrog ahead of climate change on the Senate floor; there’s no bill ready to move.

Was Graham shielding McCain? Was he looking for an excuse — in the wake of accusations that the measure would include a “gas tax” — to get out of the climate debate? Was it just getting too uncomfortable being “a caucus unto himself,” as an administration official described Graham?

I hope the weekend’s flare-up is a minor rift, quickly mended. Because the Senate badly needs more people with Graham’s combination of energy, pragmatism and courage. A Senate without a Graham in the fray would be even more dysfunctional. Which is saying something.