D.C. Del. Walter E. Fauntroy yesterday kicked off his sixth campaign for reelection as the city's nonvoting delegate, telling an enthusiastic group of about 100 supporters that his "growing seniority" in Congress has made him a valuable asset to the city.

"I was opposed to the seniority system when I first went over to Congress, but the longer I stay, the more I like it," Fauntroy told the group, which gathered for the opening of his storefront headquarters at 711 13th St. NW. "Do you want somebody else to have to fill the position and start from the bottom?"

Fauntroy recited a list of accomplishments during his 11 years in Congress, but said "the most painful irony" of his tenure was what he called "a virtual news blackout" in which his achievements and his stature in the House receive more publicity in other cities than in Washington.

"The fact is, I have become an invisible man," he said.

Fauntroy said his role as chairman of the Congressional Black Caucus and chairman of the House banking subcommittee on domestic monetary policy have given him substantial news coverage in many cities, but not locally.

He said that a federal tax and budget package which he prepared--"the Fauntroy substitute"--was one of only four budgets to be debated on the House floor. But he said the plan got no coverage from The Washington Post.

Fauntroy said he has influenced other members of Congress to look favorably on legislation affecting the city. "Only two bills in 700 have been rejected in Congress. . . . If you can't bat a thousand, well, .997 is pretty good," he said.

This year, Fauntroy is facing an active challenger in the Democratic primary for the first time since 1971--at least he was facing a challenge until Wednesday, when his would-be opponent, Marie Bembery, failed to file nominating petitions by the 5 p.m. deadline. Bembery was barred from filing by the D.C. Board of Elections and Ethics when a campaign aide who was holding the majority of her petitions failed to step inside the room before the doors closed for the day. An appeal is pending.

While not mentioning her by name, Fauntroy made repeated references to Bembery's having lived in Washington only three years, contrasting that to his much longer span of civic involvement and civil rights activity. "I coordinated the Selma-to-Montgomery march. You remember that?" he asked.

Robert Washington Jr., a lawyer and former D.C. Democratic State Committee chairman who is heading Fauntroy's finance committee, said the fund-raising target is $75,000, a figure he called "bare bones" for a congressman.

Although Bembery is not believed to be starting her campaign with widespread support, Fauntroy's campaign spokesmen said they are taking her challenge seriously.