Some funny things seem to be happening to union supporters on their way here to the AFL-CIO's Sept. 19 Solidarity Day demonstration against the Reagan administration's economic and social policies.

Those who had planned to fly are taking buses, cars and trains because of the air traffic controllers strike, now in its second month.

Those who had planned to stay overnight in Washington's union-operated hotels may be out of luck because of a possible strike Sept. 16 by Local 25 of the Hotel and Restaurant Employees and Bartenders International Union, AFL-CIO.

Local union leaders attending the 13th biennial convention of the Maryland State and District of Columbia AFL-CIO in Baltimore last week acknowledged that they are facing some logistical problems. But they were confident that they could overcome apparent obstacles and pull off one of the largest "jobs-and-justice" protests since the famed March on Washington on Aug. 28, 1963, that brought nearly 250,000 people to the city for a one-day demonstration.

"We're going to have a great turnout," said Thomas M. Bradley, president of the Maryland State and D.C. federation. "We're going to have enough people at the demonstration so that everybody in this country will know that we mean to fight against what this administration is doing to working and poor people," he said.

The Maryland-D.C. AFL-CIO, which has 521,000 members, is playing a key role in the planned protest largely because most people attending Solidarity Day activities are expected to come from the Middle Atlantic and northeastern states. The Baltimore convention was used to pump up the efforts of local union leaders to bring out as many members as possible.

Hard figures on the possible turnout -- some union leaders are predicting a total of 100,000 demonstrators, others speculate 200,000 -- were hard to come by. But the claim by Bradley and others that they have booked nearly every available Greyhound, Trailways and school bus rolling to Washington that day seems to be valid.

For example, Greyhound officials said last week that they have committed at least 12 percent, about 554 of their nationwide fleet of 4,600 buses, to Solidarity Day transportation. More buses may be added, according to Greyhound press aide Leslie White, who said the buses will be parked at the RFK Stadium once they arrive in Washington. Each bus carries 43 passengers, White said.

Trailways also reported strong bookings, and union leaders unable to get some of their people on one of the larger commercial carriers were scurrying around last week looking for school buses to do the job.

Air carriers into the Washington area lost out on the Solidarity Day transportation boon. AFL-CIO President Lane Kirkland has appealed to his 102 affiliates to support the nearly 12,000 fired members of the Professional Air Traffic Controllers Organization, many of them still walking picket lines at the nation's airports, by choosing "alternate forms of transportation." John Leyden, executive director of the AFL-CIO's Public Employee Department, said many federation unions are complying with Kirkland's request.

"Many unions had to make decisions by Aug. 19 on canceling their air charters," Leyden, a former PATCO president, said in an interview. "If they had gotten into the 30-day period between Aug. 19 and Sept. 19, under the terms of their contracts with the air lines, they would have lost money" upon cancellation, he said.

Other AFL-CIO officials said large unions such as the United Auto Workers and the International Association of Machinists were among those scrapping air charters, affecting business with Eastern and other airlines serving the mid-Atlantic and Northeast.

"You don't show solidarity by crossing somebody's picket line," said Maryland-D.C. federation leader Bradley said. Likewise, he said he doesn't expect anyone to cross picket lines that might be set up around Washington hotels if the local Hotel and Restaurant Employees union walks out Sept. 16. The last strike by Washington hotel workers was in 1946 and lasted 21 days.

There was some speculation that Local 25 leaders might postpone any possible strike action until after Sept. 19 to avoid inconveniencing Solidarity Day demonstrators or adversely affecting attendance. But Local 25 President Ronald Richardson told The Washington Post that "absolutely under no circumstances will we delay a strike" if the union doesn't have a new contract with the Hotel Association of Washington by midnight, Sept. 15.

The union is seeking a one-year, 20 percent raise for 6,000 hotel workers -- desk clerks, waiters, waitresses, maids, janitors and bellmen. The association says the wage demand is unrealistic.

Says Richardson: "The majority of the people coming in for Solidarity Day will be coming on buses and leaving on buses after the demonstration is over . . . . Should we go on strike, it'll be the kind of thing that will bring the labor movement together. Many people who had planned to stay over say they will picket with us if we're on strike. The solidarity of the labor movement does not depend on the availability of $100-a-night hotel rooms in Washington."

But, according to officials at the Capital Hilton, housing headquarters for visiting Solidarity Day VIPs, the labor movement cared enough about hotel space to rent a block of 200 rooms -- albeit with contracts that stipulate that the prospective guests won't be liable for payment on the rooms if there is a strike.

The AFL-CIO also has rented the Presidential Ballroom in the Capital Hilton for a meeting of 1,000 Solidarity Day parade marshals, scheduled Sept. 18 from 8:30 p.m. to 10:30 p.m.

"What are they going to do if there's a strike? Cross the lines? Go to a non-union hotel in Virginia? This is a kind of funny situation," a Capital Hilton official said.

Richardson said local union officials are prepared to put up as many overnight demonstrators as possible in the event of a strike.