A prominent union leader offered the District government an old-fashioned political tradeoff yesterday in the opening round of the kind of tough bargaining that is second nature in other big cities, but is new to the young home rule government in Washington.

Kenneth T. Blaylock, national president of the American Federation of Government Employees, said he would put the full weight of organized labor's lobbying power behind the city's money bills in Congress, provided city government's employes receive acceptable salary increases over the next two years.

Blaylock, who is a member of the executive committee of the AFL-CIO, said Mayor Marion Barry's proposal to withhold pay raises for the city's 31,000 workers for the next two years to save money was "unacceptable" and might be challenged in court if implemented.

In a statement that he said had the approval of the AFL-CIO leadership, he told the City Council that "we stand ready to work with you in an objective effort to resolve the financial crisis facing the city, but the proposed effort which breaks the convenant between labor, the city workers, and this council and the mayor will be met with resistance by the entire labor movement."

Blaylocks' union represents about 9,000 city workers, including all of those in the department of corrections, housing, recreation and transportation. tHe noted that President Carter had proposed a 9.1 percent pay increase for federal white-collar workers on Oct. 1, while city employes, who before home rule enjoyed parity with the Federal colleagues, might get nothing Baylock asserted that the city "cannot afford to balance its budget primarily on the backs of its workers and their families."

He said that "all 14 million" trade unionists in the country would support the city's requests in Congress for a bigger federal payment and more taxing power, but "the support of the labor movement is contingent upon your treatment of your employes."

Before home rule, public employe unions bargained with the city government over working conditions and hours, but salaries were decided by the president and Congress. Now, under the city's 1979 Personnel Act, the workers no longer are ensured of Federal pay comparability, but they are guaranteed the right to collective bargaining over their wages.

They are prohibited from striking. Geraldine Boykin, executive director of Local 20, American Federation of State County and Municipal Employees, said that "nobody is talking about a strike," but she said that "they can't be made to work without money."

Her union respresents about 16,000 city workers.

She and Blaylock were testifying before the City Council's first hearing on the comprehensive long-range financial plan that Barry introduced in July.

Chairman Arrington Dixon, who presided, said that since Barry offered his plan, which is based on a combination of austerity in city affairs and congressional help on long-range fiscal problems, the council has seen no implementing legislation, no information on current spending cutbacks, and no assurance that Barry had congressional support for his requests.

The only certainty, he said, was that on Oct. 1 the council will get the mayor's proposed budget for the 1982 fiscal year -- a budget that contains no money for salary increases. "The council has before us only a phantom plan," Dixon said, and "I am concerned that we may be getting a phantom budget."

Most of the testimony at the day-long hearing covered territory by now familiar: doubts about the validity of the mayor's figures and skepticism about his plans for elminating the city's cumulative deficit. The new element is the sudden urgency of settling on the wages to be paid the city's workers over the next two years, an issue that will dominate District Building politics for the rest of this month.

Philip M. Dearborn vice president of the Greater Washington Research Center and an acknowledged expert on District money matters, said this was "the most difficult and pressing question the city is going to have to face."

He said there is "clearly no funding" available to give city workers the same 9.1 percent that federal workers have been promised, and suggested that taxed might have to be raised to finance pay increases in subsequent years.