While the republicans, led by their standard-bearer Ronald Reagan, were advertising their unity on Capitol steps yesterday, six members of an endangered species, the progressive Democrats, gathered in McLean to fight that Republican brotherhood.

"It's a tough year for everyone," said Morris Udall. "My opponent, a millionaire, born-again, real-estate executive is shooting for $400,000. He's got coal money, oil money. It's just not easy."

Udall was voicing the down-but-not-beaten sentiment of five of his colleagues on the House Interior Committee. The group, Udall of Arizona, Robert Eckhardt of Texas, Raymond Kogovsek of Colorado, Peter Kostmayer of Pennyslvania, Bruce Vento of Minnesota and James Weaver of Oregon, have all been targeted for defeat by the so-called New Right. Last night they were the center of attention at a 100-person fund-raiser at Virginia State Sen. Clive DuVal's estate. The guests, who included LaDonna Harris, the vice-presidential candidate of the Citizens Party, and former State Department spokesman Hodding Carter, were invited by the Council of Energy Resources Tribes.

The raucous unity of the Republicans yesterday hadn't impressed the Democrats. "That was a lot of hoopla. I watched George Bush play racketball," said Weaver nonchalantly.

One reason for the rally was to show that the Reagan-Bush team could work effectively with Congress. Kogovek has mixed feelings about carter's rapport with Congress: Carter vetoed a bill of his which would have given the Ute Mountain Tribe compensation for some of their land.

"I hope it was just miscommunication," said Kogovsek, who reintroduced the bill yesterday. "That's what Frank Moore told me. I think the Justice Department was afraid we might set a precedent on royalties. It was $6 million in royalties on about 3,000 acres of land. Overall I think Carter's communication with Congress is improving."

Kostmayer's opponent, Jim Coyne, was at the Capitol yesterday. "I was back home," said Kostmayer. "But one of my staff spied him and yelled out, 'How do you feel about the 55 m.p.h. speed limit now?"l You see, he just lost his license for speeding and the Republicans are for eliminating the 55 speed limit.

"He's outspending me 3 to 1. I'm optimistic, but I am worried about the others on the committee; they are more endangered than I."