Alea iacta est. That’s what Julius Caesar proclaimed as he crossed the Rubicon River in 49 B.C. It means, “The die is cast.” By crossing the Rubicon with his army — against Roman law — Caesar guaranteed a head-on conflict with the overconfident Roman ruler Pompey. Outnumbered, Caesar was presented with the choice: win or die.

The recent special election in the 26th congressional district of New York was a political Rubicon. The Democrat, Kathy Hochul, ran against the Republican budget, specifically Rep. Paul Ryan’s plan to save Medicare by turning it into a voucher program starting 10 years from now (excluding all current beneficiaries).

The Republican, Jane Corwin, said she supported the plan and then spent much of the campaign defending it with all of the verbal dexterity of a contestant in a cracker-eating competition.

It’s difficult to exaggerate the gloating and glee from Democrats about their triumph. Their takeaway: Democrats can win if they demonize the Ryan plan and run ads (or allow third-party groups to run them) showing old ladies being flung from cliffs like Spartan infants.

There is certainly good reason to believe that Hochul’s so-called “Medi-scare” tactics made the difference. It was the top issue for nearly a quarter of voters. And while most of them may well have voted Democratic anyway, the simple fact is that Hochul won a single-issue campaign in a district that shouldn’t have elected a Democrat at all.

Republicans console themselves by noting that she won with only 47 percent of the vote, and a Democratic gadfly who bought the Tea Party label garnered 9 percent of the vote. Meanwhile, Corwin simply wasn’t a very strong candidate. Karl Rove points out in the Wall Street Journal that Hochul received merely one more percentage point than Barack Obama did when he lost the district in 2008.

Even Paul Ryan says that aside from the Democrats’ lies and distortions about his plan, Hochul’s victory “shows that a Democrat running as a Tea Party candidate dumping a couple million dollars in the race is going to have an effect.”

Fine, fine. For the sake of argument, let’s stipulate that’s all true. So what?

The simple fact is that the Democrats have their battle plan. It’s going to be Medi-scare every day in every way for the next 17 months. They are on autopilot. They are committed. Their die is cast. They have crossed their Rubicon. They have no desire to defend Obamacare, high gas prices, high unemployment, and a third Middle East war. They want — no, need — to be on offense because they have so much they cannot defend.

The question now is, “What are Republicans going to do about it?” Are they going to play the role of Pompey, the dissolute leader who didn’t want to fight? Or will they don Caesarian robes and join the battle head-on because they know they have nowhere to retreat? That is the political choice for the GOP: Win or die.

There’s an entirely plausible case to be made that the GOP bravely blundered in passing the Ryan budget. I don’t agree with that argument. But again, so what?

A surefire way for that claim to be proven true is for Republicans to start hemming and hawing and apologizing for what they’ve done. Look, the House of Representatives passed it with a near-unanimous vote among Republicans. Forty Senate Republicans voted for it as well. Republicans can’t run from that, so they shouldn’t try.

The one advantage the outnumbered Caesar had was that he and his battle-tested forces understood that there was only one solution to their plight: victory.

The battle-tested Republicans have the same suite of options. And they are battle-tested. Last November, they won sweeping victories in the midterm elections. How? By focusing first and foremost on the Democrats’ failures.

For instance, the Democrats have a plan too. It’s the Status Quo-Plus. It involves letting Medicare continue to spiral out of control, consuming our budget until it becomes necessary for an unelected chamber of health-care bureaucrats to impose draconian cuts. Actually, Democrats have two plans. That was the Obama plan. There’s also the Harry Reid plan, which involves lawlessly refusing to pass a budget for close to 800 days.

The GOP does need to be more optimistic and pro-growth. It can’t just sell reality-based pain when the opposition is selling the cheap lies of deceit.

But more than anything, Republicans need to realize that the die has been cast. All that is left for them is to decide whether they will play the role of Pompey or of Caesar. 

— Jonah Goldberg is editor-at-large of National Review Online and a visiting fellow at the American Enterprise Institute. You can write to him by e-mail at JonahsColumn@aol.com, or via Twitter @JonahNRO. (C) 2011 Tribune Media Services, Inc.