"This is the best shot we ever had," one Washington business community leader says in describing the long drive by city leaders to construct a giant convention center near Mt. Vernon Square.

Today that effort culminates in a sudden-death playoff on Capitol Hill. The local community's strategy of putting all of the weight it can muster behind convention center one idea - a $110 million downtown complex - is on the line.

Members of a Senate-House conference committee assembled to resolve differences betweeen the two bodies of Congress on D.C.'s budget for fiscal 1978 are the players in today's game. But the people they represent from Vermont to the West Coast won't be affected much by the outcome.

If the committee approves the $27 million of startup costs for the project, proposed by an elected city government and endorsed by the House, there would be potential winners and losers among District residents or owners of the businesses in the Mt. Vernon Square area and property owners in general.

And if the Senate conferees prevail - backed by a 65-to-25 vote on Oct. 4 that turned down a specific amendment in support of such spending - the impact again will be in the local community.

Large organizations holding conventions need a meeting center larger than now exists here so one will be built - perhaps in the suburbs, where residents don't need approval of Congress for local projects.

Some local business leaders who support the District convention center are prepared for defeat today. "Tough chance," notes one.

But there is a great deal of quite confidence, too. "It all depends upon the strength of [Pep. William H.] Natcher," says a business community leader who is counting on the Kentucky Democrat and his House colleagues to win on the convention center issue.

That is a tribute with some irony, because it was Natcher who three years ago blocked a convention center after Senate approval.

Congress itself authorized a convention center in 1972 after many years of lobbying by local interests. But financing of the proposed complex must be reviewed by the appropriations committees.

Natcher's foes on the convention center issue this year are headed by Sen. Patrick J. Leahy (D-Vt.), chairman of the District Appropriations Subcommittee, who last week predict confidently that Senate conferees would hold firm and that the center is dead.

Since that forecast, chief spokesmen for local businesses have not been in contact with Leahy's Senate colleagues, preferring to let Natcher do his own work. Clarence Arata, executive vice president of the Metropolitan Washington Board of Trade and a prime leader of pro-center forces, even has left town.

One measure of confidence among local business leaders is the absence of any alternative to the Mt. Vernon Square plan, designed to generate an estimated 4,000 new permanent jobs for the city's weak economy.

If the vote today goes against the convention center, there is no "fall-back position" no compromise location or new method of financing. What would follow is a period of finger-pointing to assess blame. There would be a regrouping of convention center planners. Prince George's County leaders might dust off plans for a convention center of their own.

But Joseph B. Danzansky, chairman of Giant Food, Inc., head of a Board of Trade task force on the D.C. center and always an optimist about the Washington area, thinks the Prince George's plans can stay on the shelf.

"I have every confidence in the conference committee. I really don't think any of them want the city to remain in a colonial capacity . . . We're only asking for the right to borrow money," he says.

Although Danzansky has avoided any lobbying effort with members of the Senate since that body's defeat of convention center spending he says "some prayers" have been said.

"When faced with the realities of cutting this city off from its future, the good will prevail," is all Danzansky wants to say.

AROUND THE BELTWAY - At last, the public relations Firm of Hill and Knowhow, Inc., is publishing a new version of its "Directory of Key Government Personnel," a pocket-sized booklet of telephone numbers that is one of the 50-year-old firm's best read products. The new book has 1,793 characters and no plot. H&K has scheduled a wine and cheese party Oct. 20 at the National press Ballroom in honor of the book and the firm's birthday.

Marriott Corp. declined comment on a reprpreprepreort in London the Washington firm may buy an uncompleted Kings Beach Hotel there for $13.2 million and that it will spend an equal amount to complete the structure . . . Marriott says it comments on such projects only is a contract is signed.

MARYLAND, TOO - Business leaders in the State of Maryland are concerned about trends in their state - particularly the gradual shift from a strong manufacturing base centered at Baltimore [WORD ILLEGIBLE] oriented economy where government employment is becoming a key factor.

The state's business community, however, has not been among the most aggressive in the nation. Now, the Maryland Chamber of Commerce is moving to change that image.

Noting reports that the state lost 411,000 manufacturing jobs between 1970 and 1975, chamber president, Eugene BB. Moore Jr. says his group must make state residents aware of the need to encourage existing businesses to remain and to expand, while encouraging national and international firms to locate there.

As an initial step "to halt further erosion of our state's economy," a new vice president for member relations and communications has been named. Moore says. He is Robert P. Carey, for the past 11 years public relations manager for Western Electric Bell Systems sales division in [WORD ILLEGIBLE], Md. Carey is a Baltimore native, is a graduate of the University of Maryland.

Washington lawyer Meyer Feldman has increased has ownership of National Savings & Trust Co. stock to 97,000 shares 24,000 over what already was the largest single holding . . . NS&T has about 458,000 shares outstanding.