But for voters of all stripes, Tuesday’s primaries should illuminate the growling face of a new fringe in American politics — and provide the incentive for level-headed voters to become enthusiastic about the midterm election.

Republican leaders have to decide if they want the tiny fraction of furious voters who have showed up at the primary polls to steer them into the swamp for years ahead. They have a chance to repudiate the worst of the Tea Party crowd and show that they can govern without appealing to the basest political instincts. So far, they have preferred to greedily capitalize on the nuclear energy in the land without considering its destructive effects.

Democrats, especially beleaguered incumbents and the White House, need to counter the toxic message of the Tea Party so voters have an alternative.

For both parties and certainly the broad swath of independent voters, defeating this new crop of Tea Party nominees has become imperative to avoid the sense of national embarrassment from each divisive and offensive utterance, each wacky policy proposal.

Take the new Republican nominee for United States senator from Delaware, Christine O’Donnell. She founded a group called the Savior’s Alliance for Lifting the Truth, with a curious focus on sexual purity, and claimed there was scientific evidence that God created the world in six 24-hour periods. She lied for years about being a graduate of Fairleigh Dickinson University, having earned a degree only in recent weeks, 17 years after she left campus. She has no steady source of income and has a substantial trail of unpaid bills, battles with the Internal Revenue Service and questionable use of campaign donations for personal expenses.

Ms. O’Donnell defeated Mike Castle, a veteran congressman and example of the moderate and conciliatory approach that Northeast Republicans once brought to Washington. Her campaign ridiculed him for being 71 years old with a history of heart problems. Ms. O’Donnell called Mr. Castle “unmanly.”

Or consider Carl Paladino, the Republicans’ new nominee for governor of New York, who has transfigured the state’s justifiable disgust with Albany into a malevolent snarl at the world. It is one thing to promise to shake up state government; it is very much another to thuggishly proclaim that he intends to clean up Albany “with a baseball bat” and turn the Assembly speaker, Sheldon Silver, upside down to get his blood flowing and then send him “to Attica.” This is the man who has vowed to send welfare recipients to state prisons to pick up their checks and be given lessons in hygiene. He has defended an ally’s comparison of Mr. Silver to Hitler or the Antichrist and is known for forwarding e-mail messages to friends with racist or pornographic images.

In both cases, the Republican establishment did everything possible to avoid having the party be represented by these two, lest the link to the Tea Party become evident. Karl Rove, long the party’s tactical mastermind, dismissed Ms. O’Donnell as “nutty.”

But, in fact, the party’s hopes for retaking Congress are deeply bound up with the fate of Tea Party candidates across the country, and the party’s leaders have done little to distance themselves from the extremism that now constitutes mainstream conservative policy.

When the House Republican leader, John Boehner, voiced a possible compromise on tax cuts, he was immediately shouted down by other party officials and pilloried as weak by right-wing blogs. Mr. Rove noted that Ms. O’Donnell is unlikely to win in November, possibly preventing the Republicans from taking over the Senate. He is now a pariah himself in those same circles.

On Wednesday, Mr. Boehner invited Tea Party activists to help “drive the debate” in Washington and shape the legislative agenda. That invitation act should be a dose of adrenaline to dispirited Democrats, independents and mainstream Republican voters who had not fully grasped the stakes in November’s election.