Politics, especially Idaho politics, is in Larry Craig’s DNA.

He has represented Idahoans in Congress since 1981. He was elected to the state Legislature when he was 29. He was student body president at the University of Idaho, and before that he was state president of Future Farmers of America. .

It’s a lifelong passion – and it’s clouding his judgment. Otherwise, he would do the reasonable thing and call it quits now. That would be best for Idaho as well as for Craig.

This week, a Minnesota judge flatly denied the Republican senator’s attempt to withdraw the guilty plea he entered in August to a misdemeanor charge involving his arrest in a Minneapolis airport restroom. That was to have been the determination that would prompt Craig to quit right away, allowing Gov. Butch Otter to name a replacement for the remaining 15 months of Craig’s term.

Craig is reneging on his promise, saying he intends to serve out his term, partly so he can defend himself against charges being investigated by the Senate Ethics Committee.

Although the veteran politician insists he can still represent constituents effectively, the evidence contradicts him. Shortly after Craig’s embarrassing legal problems came to light a month ago, his Republican colleagues stripped him of his prized status as ranking minority member on three Senate committees. Several Republican senators called on him to resign, including Mitch McConnell, who called Craig’s actions “unforgivable.” This week, Nevada’s John Ensign, chairman of the National Republican Senatorial Committee, called on Craig to keep his word.

That environment doesn’t promote effectiveness.

Meanwhile, valuable time is slipping away for Craig’s successor to start learning the job, meeting his colleagues and, as Craig himself would concede, begin building the seniority so important in the Senate.

Already, prominent Republican Sens. John Warner of Virginia, Pete Domenici of New Mexico and Chuck Hagel of Nebraska have announced they won’t seek re-election next year. In other words, in both the Senate and its Republican caucus, a new Idaho senator taking office in the next month would gain an edge in the influence he could use on behalf of Idahoans.

By prolonging the episode, Craig denies his constituents an advantage while treating them to unending embarrassment.

Had he resigned in August, Thursday’s court ruling would have passed with little notice. Instead, it revved up discussion about fellow Republicans’ disapproval and Craig’s refusal to keep his word. And for those who pay close attention to such matters, the findings issued Thursday by Hennepin County District Judge Charles Porter expose in meticulous detail how feeble Craig’s claim is that his June 11 arrest was just an innocent misunderstanding, attributable to a wide stance and political panic.

For a third of a century, Larry Craig has been asking Idaho voters for the privilege of serving them. For a third of a century, they have obliged. Now, politically hobbled by his own mistakes, Craig can honor their trust by stepping down.

Not at the end of this term, now.