Proponents of legalized gambling in Washington, stung by a barrage of 11th-hour attacks on Tuesday's referendum that would sanction betting on jai alai, dog racing and a lottery, dug in yesterday to fight off what gambling foes are convinced is a marked shift in opinion against the measure.

The gambling forces, fueled by another $20,000 in donations to their campaign in the last week, started telephoning about 15,000 voters they believe support gambling to again urge them to vote for the proposal and scheduled additional radio advertisements.

In addition, Brant Coopersmith, chairman of the umbrella group promoting legalized gambling in the nation's capital, said for the first time that if the initiative is passed Tuesday he would support a key change that would call for the D.C. mayor, not a new and powerful Gaming Control Board, to propose who would get an estimated $35 million in annual gambling revenue.

Coppersmith rejected the contention of gambling opponents that momentum had shifted against the measure, an all-or-nothing proposal drafted by gambling supporters. But he added, "We realize the enemies out there are real."

Coppersmith was paraticularly incensed about a broadside leveled against the proposal by U.S. Attorney Charles F. C. Ruff, who said the 43-page bill is so "loosely drafted" that pari-mutuel wagering on dog racing and the fast-tempo Spanish sport of jai alai could not be properly policed and might attract "organized criminal elements" because of the large sums bet on the sports.

"We've been made to look procriminal," Coppersmith complained.

But in addition to Ruff's highly unusual statement on Friday, two of the three major network television stations here have editorialized against passage of the measure, as have The Washington Star in late March and The Washington Post in today's editions.

Even Joseph L. Rauh Jr., the prominent civil rights attorney whose name appears on a brochure promoting passage of the referendum, wrote in a letter to The Post that he now supports creation of a city-run lottery, but not legalization of jai alai and dog racing, and will so mark his ballot.

"The wind has shifted," said the Rev. John D. Bussey, chairman of the antigambling group headed by a small group of poorly financed black Baptist ministers. "It's a good wind. The bill itself is killing it. If things keep going this way, we'll win.

"People who had not understood what was in the bill are now asking questions," he said. "It's a good sign."

But Coppersmith and other gambling supporters said voters should not be led to believe that if Tuesday's referendum is defeated, a more limited gambling measure, perhaps without the controversial jai alai and dog racing provisions, would be voted on at a later date.

"I hope people realize this will be the last time for a long time to get gambling," he said. "If people want gambling, they better vote for this."

Coppersmith said that any provision of the bill could be amended later by the City Council, although it is unclear exactly how soon after the referendum. In any event, he said it would take at least a year to make preparations for legalized gambling in Washington, giving the council plenty of time to make suggested changes.

A majority of the council opposes gambling and last year refused to vote on a proposal to have a nonbinding advisory referendum on whether to have legalized gambling here.

Bussey scoffed at Coppersmith's suggestion that any provision critics don't like could later be amended.

"The people can see through the fallacy of voting for it with the hope it will be amended," Bussey said. "If this thing is passed, I think we'll have a difficult time getting it changed."

An antigambling rally was held late yesterday afternoon at the District Building. About 300 persons were counted at the gathering, organizers said.

The hour-long rally had the air of a revival with hymn singing, hand clapping and prayers. Placards, one reading, 'There's enough corruption in D.C. Vote No on Initiative No. 2," also were waved in air for passing autos and theater-goers across Pennsylvania Avenue.

The Rev. Andrew Fowler, paster of the Capitol View Baptist Church and an organizer of the rally, said his group had sought a racially mixed group of speakers and crowd and was pleased with the turnout. He urged the crowd to carry the campaign to religious services today.

Both supporters and opponents of the measure say that one of the keys to the outcome of Tuesday's election will be how substantial a majority votes against the initiative in Ward 3, the affluent voting district west of Rock Creek Park.

Martin E. Firestone, a Washington lawyer and one of the authors of the gambling measure, said that it is possible that an overwhelming majority in Ward 3, especially if it is a heavy vote, could spell defeat for the measure even if it is approved in all of the city's seven other wards.

Firestone predicted that the initiative would lose in Ward 3 by about 10 to 15 percentage points.

There is at least one electoral precedent for Firestone's fears. In a special 1977 at-large City Council race, Barbara A. Sizemore, the controversial former D.C. school superintendent, was swamped in Ward 3 and lost the election even though she captured a majority of the vote elsewhere.

"I see a kind of elitism and paternalism in Ward 3 that doesn't exist elsewhere, that they know what's best for the poor," Coopersmith said.

Part of gambling proponents' fears about the turnout in Ward 3 are generated by the fact that nearly half of the city's 22,488 registered Republican voters live in Ward 3. Many of them are expected to vote in Tuesday's GOP presidential primary and at the same time vote "No" in the gambling referendum.

"I have not talked to a single Republican who's going to vote for it," city Republican party chairman Paul Hays said this week.

Gambling supporters say they expect to roll up their biggest majorities in the heavily black wards -- 4, 5, 7 and 8 -- that abut Prince George's County, where many D.C. residents troop almost daily to play Maryland's popular legal numbers game.

Even a gambling opponent, City Councilman John Ray (d-at-large), says, "From what I can tell, the yea vote is out front by a fairly comfortable margin, at least 10 percent" in Wards 4, 5, 7 and 8.

He said he thinks the outcome will be much closer in the inner city wards -- 1, 2 and 6 -- areas where many affluent whites are rapidly moving into previously majority black neighborhoods.

The gambling supporter's $103,000 campaign has been heavily financed by the Washington Jai Alai Corp., which wants to open a jai alai arena here, companies that manufacture lottery computers and tickets, and D.C. liquor dealers, who want to get sometimes lucrative lottery ticket sales commissions like their competitors in Maryland. But in the last week, a new group called the Columbia Kennel Club, which wants a greyhound racing track here, pitched in $10,000 toward the progambling effort.

Bussey said the ministers' group may end up raising $18,000, compared to eventual total spending of $115,000 for the progambling forces. The ministers' fund-raising effort has been hampered somewhat by a dispute with the city's campaign finance office.

Campaign finance officials had to issue subpoenas to get some records concerning the ministers' fund-raising contract with James Rinker, a former congressional aide starting his own political consultant's firm.But Lindell Tinsley, the acting city campaign finance director, said the investigation was now completed after the documents were turned over.