In its first major foray into the federal government sector, the International Brotherhood of Teamsters forced the American Federation of Government Employees (AFGE) into a runoff election yesterday for the right to represent the statisticians, demographers, researchers and clerks at the U.S. Census Bureau.

A runoff is necessary because neither organization got a majority of the 1,563 ballots cast. The AFGE, which has represented the bureau's 3,100 employes for two decades, received 739 votes; the Teamsters, 538 votes, while 286 employes voted for the option of no union.

"We saw a strong no-union vote and were disappointed," said AFGE national representative Rickman Seidel, blaming a recent management letter to employes that he claimed emphasized the no-union option. Seidel said the next election likely will be held in two months.

Although the Teamsters already have made inroads into Washington's city government, yesterday's election was the first involving Teamsters and an entire agency of the federal government. It reflects the continuing push of the nation's largest union to diversify and attract professional workers, particularly in government.

About 300,000 of the Teamsters' 1.7 million members are local, state and federal government workers, with the smallest fraction involving federal employes on individual military bases in Oklahoma, Louisiana and Florida.

"This is just the beginning," said Louis Richard, a Teamsters organizer who led the campaign at the Census Bureau. "At least four other large agencies within the federal government are waiting to talk to us."

The fight began in January with an election petition containing the signatures of 1,150 census employes seeking to join the Teamsters. In recent weeks, workers at the bureau's sprawling complex in Suitland have been bombarded with slick leaflets, Teamsters key rings, AFGE carnations and other gimmicks. At noon yesterday in the agency's basement cafeteria, an elderly man in a straw campaigning hat dispensed the last of 2,500 Teamster coffee mugs from a grocery cart decorated with the sign, "It's time for a change."

The Teamsters used that slogan as the cornerstone of their campaign, criticizing the AFGE as weak and promoting themselves as tough, aggressive bargainers with the clout of an international organization behind them.

AFGE workers, on the other hand, have touted their record of accomplishments -- flexible work schedules, a health plan that received high ratings by the consumer magazine Checkbook, career development programs and a negotiated grievance procedure. They have charged the Teamsters with making empty promises and denounced their lack of experience in the federal government.

The Teamsters' long-running image problems, including instances of corruption and criminality, have also surfaced. One AFGE leaflet bore the heading, "Are the Teamsters honest?"

"The fact is that the Teamsters don't have a record that shows effective representation in the federal government, and we do," said Bill Milton, who heads AFGE Local 2782. "This experience will only make our union stronger."

The Teamsters, who do not belong to the AFL-CIO, have been successful in a number of so-called raids on AFL-CIO unions such as AFGE and the American Federation of State, County, and Municipal Employees (AFSCME). Last year, the D.C. school system's 2,500 blue-collar workers voted overwhelmingly to join the Teamsters, dumping AFSCME after 15 years. In 1985, the Teamsters won an election to represent 2,400 District prison and jail employes; however, those employes recently completed a petition with 976 signatures calling for a new election that could reinstate the ousted AFGE.

At the Census Bureau yesterday, the union election was the talk of the offices, hallways, and rest areas. In the cafeteria, Michael Wolf, a statistician with a master's degree, said he had difficulty envisioning himself as a Teamster, given the blue-collar image.

"I have no fault with the AFGE," said Wolf, who is not a member. Although AFGE represents the entire bureau, membership is optional under federal labor law, and only 341 census workers have joined the union.

But a 20-year employe in administrative services, also not a union member, said his colleagues had criticized the AFGE as "too complacent."

Many people, however, seemed to share Steve Smola's attitude. They like their jobs. They don't belong to the union. They'll be glad that the election is over.

"I'm bored with the whole thing," said Smola, a statistician. "I'm not a union person at all. As a matter of fact, I don't like the NFL strike either."