D.C. City Council member Arrington Dixon summoned reporters to the Council Chambers for a rare Saturday morning press conference yesterday to announce his endorsement of Council Chairman Sterling Tucker for mayor in 1978, even though Tucker has yet to officially announce his candidacy.

Dixon would not stay if he has been assured that Tucker is going to run for mayor. Dixon did not rule out running for mayor himself. And he refused to say if he planned to run for Council chairman (as Tucker's running mate), as has been widely speculated in local political circles.

With still nearly a year to go before the Democratic mayoral primary, the winner of whom is almost assured of being the city's next mayor. Dixon's vague and apparently oddly timed annoucement seemed somewhat strange.

But it was perfectly in keeping with the subtle bump-and-run style of politics that is taking place among the city's Democrats these days in the face of what is increasingly being viewed as an unavoidable mayoral showdown between two strong and popular party leaders - Tucker and fellow Council member Marion Barry.

For weeks, party leaders and Tucker supporters including Del. Walter E. Fauntroy (D.D.C) have been placing increasing pressure on Barry to abandon his plan to challenge Tucker for mayor and thereby Party's ranks. Fauntory and others would prefer for Barry to be TUcker's running mate and seek the COuncil chairmanship.

Dixon's announcement was seen by several leading Democrats yesterday as the first indictation that the mayoral scuffling has turned the corner because Barry does not plan to withdraw his challenge to Tucker. Thus leading party members are beginning to take sides.

In addition, it appeared as if Dixon's announcement was couched in terms that would not only give Barry a nudge not to buck Tucker, but would also announce that if Barry is not willing to run with Tucker, Dixon is.

"In a word," a ranking city Democrat said afterward. "Arrington is saying. Marion, you're the one (for chairman). But should Marion not decide to do it - which it is obvious he will not - Arrington has painted a decription of the next Council chairman that best fits himself."

Dixon said yesterday that, "It is still my earnest hope that I shall have the occasion to endorse a man I admire a great deal. Councilman Marion Barry, for the position of chairman, for I believe he has much to bring to this vital and significant office."

But immediately afterward Barry said, "I feel all that is very strange and very interesting, but it is not going to affect my plans. I'm not running for chairman, and I've told Arrington that."

Dixon, the 34-year-old Council member from ward four in upper Northwest Washington is in his second term on the Council. He is close to both Tucker and Barry, and his wife. Sharon Pratt Dixon, is also active in city politics, serving as the city's Democratic national committeewoman.

In his announcement yesterday, Dixon pegged his support for Tucker in large part on the very same rationable that Fauntroy and other Tucker supporters have used to try to discourage Barry, namely that Tucker is older, more experienced and has served his time as Council chairman.

Dixon also made an oblique reference to one of the most recent uncertainties about Barry, namely that the former community activists - who recently referred to the chairman of the powerful Senate Appropriations Subcommittee for the District as senator from a "rinky-drink state" - may not have mellowed enough to serve as mayor.

"The leadership our city most assuredly needs and most unquestionably deserves . . ." Dixon said, "is a leadership that is characterized with aggressiveness - but an aggressiveness that is tempered with a careful, thorough and deliberative approach.

Dixon said he hoped his announcement would spur other city Democrats to "make public soon their choice." His statement made no mention of Mayor Walter E. Washington, who most members of the D.C. Democratic State Committee have written off, but some city businessman have not abandoned. The mayor has not said he will seek re-election.

With the pressure on him growing, Barry's supporters are now for the first time saying publicy that he does intend to run for mayor and, if a city, law that would require him to relinquish his at-large Council seat to be a mayoral candidate is not overturned, he will quit the Council.

"What Marion is saying loud and clear is that I'm running for mayor. If the law doesn't change I will resign and run for mayor. That's what he'd like to say but he can't say it," David Abramson said last week.

Abramson's firm. Abramson-Himmelfarb Inc., recently completed a survey of 1,000 city residents to determine attitudes about city issues and possible mayorable candidates. Abramson will not give details of the poll, but said because more than one-third of those questioned were still undecided on their choice for mayor, the poll itself is somewhat inconclusive.