Jack Long made a boo-boo. A few weeks ago he was practicing law in Burlington, Vt., and increasingly distressed because there was no Democrat in the race for Vermont's lone House seat.

So he decided to run. After all, he was a party member in good standing and a former state commissioner of environmental conservation. He thought he had a good chance as the moderate alternative to independent incumbent Rep. Bernard Sanders, Congress's only socialist, and conservative Republican challenger Susan Sweetser, a favorite of the House GOP leadership.

Think again, Jack. The evening before his June 27 announcement, he got a call from Rob Engel, political director of the Democratic Congressional Campaign Committee, the folks who coordinate the party's national campaign strategy. Engel was not pleased.

It turned out that the DCCC was totally committed to Sanders, who votes with the Democrats most of the time, and had no interest in one of their own muddying the waters and maybe handing the election to Sweetser. The Democrats are fiercely intent on regaining control of the House, and every seat counts. No time to back quixotic losers.

Engel explained all this to Long in what both men described as a less than cordial exchange. In fact, Long was still in shock a week after getting blitzed by "some Democratic Beltway bureaucrat with an abundance of arrogance."

Nonetheless, Long said, he had decided to ignore the purported insult and go about his business. But then the Rutland Herald a couple of days later quoted Engel as saying Long "may even be a plant by Susan Sweetser."

"It's preposterous," Long said. "I feel like I'm caught in a Kafka play of some sort." So he went public with his side of the story and wrote a letter to President Clinton, asking that Engel be fired.

Over at the DCCC, Engel is not apologizing. The Democrats are out to defeat Speaker Newt Gingrich (R-Ga.) and "his extreme Republican agenda," he said, and "we will let nothing stand in our way.

"The truth about him {Long} is that he calls himself a Democrat, runs for office and helps Susan Sweetser," Engel said. "He is the only candidate in the country who put his personal agenda before the agenda of the national party," he added. "That is not our definition of a Democrat."

That seems obvious, given the party's support for Sanders, a man who in 1992 denounced both the Democrats and Republicans for "grossly inadequate" tax proposals and continuing to "provide more and more tax breaks for the rich."

In fact, Sanders is the real deal, a lefty who time and again has advocated radical income redistribution through the tax system. He is an authentic tax-and-spend guy, and proud of it.

Sanders's wife, Jane, who acts as his campaign press secretary, says her husband has never been a member of the Socialist Party or the Socialist International, preferring to remain independent while advocating a line similar to that of mainstream European social democratic parties.

Sanders has never gotten any money from the Democratic Party, and was rebuffed as a dangerous pinko when he tried to join the Democratic House Caucus after his first election to Congress in 1990.

But that was then, and this is now: "These are unusual times," Jane Sanders said. "Our country's at a crossroads." And the Democratic Party is pretty sure what fork Sanders is taking.

"Bernie Sanders has stood with us for a year and a half since 1995, and against Newt Gingrich," Engel said. The enemy of my enemy is my friend, and for this reason, if for no other, the DCCC wants Sanders to win in November.

And the friend of my enemy is my enemy, which is the pigeonhole where the DCCC has stuffed Long. Nevertheless, Long received a gush of support after the Engel shootout, and even got a $100 check in the mail. He figures he needs about 3,000 more of these to give it a reasonable go.

Sanders, who has never before received any attention from the national Democratic Party, has welcomed Long to the race, and Jane Sanders suggests that Long could do as much damage to Sweetser as to her husband.

So far, Long doesn't appear to be doing much damage to anyone. A Becker Institute poll in late June of 400 likely Vermont voters put Sanders far ahead with 52 percent to 35 percent for Sweetser and 4 percent for Long.

"The conventional wisdom is that Mr. Long will hurt us, but in Vermont voters either love Bernie or don't like him at all," Jane Sanders said. "The people who vote against him all the time now have two alternatives."

Sweetser, one imagines, still likes the odds. CAPTION: JACK LONG