Is embattled Teamsters Union losing clout?

The International Brotherhood of Teamsters (IBT), the country's largest union , is encountering growing problems in its industry and within its ranks.

Its president, Roy L. Williams, is on trial in federal court in Chicago on charges of a conspiracy to bribe US Sen. Howard W. Cannon (D) of Nevada. A jury has been chosen, and the trial of Mr. Williams and four co-defendants is under way after 17 months of delays. It is expected to drag on for eight to 10 weeks, with a possibility of continuing legal maneuvers and appeals beyond that.

However, there appears to be less concern within the Teamsters over the eventual fate of the union's $225,000-a-year president than over problems that are only tenuously related to Mr. Williams.

The 1.8 million-member IBT, once considered beyond challenge in its industry, has lost considerable ground. The federal government has deregulated the trucking industry, and union employers are losing business to more than 6,500 new firms operating without labor contracts and with 20 to 30 percent lower costs.

Once-solid national trucking contracts are in danger as employers and union members show growing dissatisfaction with current labor agreements. Tougher bargaining in the future could bring threats of strikes.

Many union members are unemployed or working for nonunion companies. IBT, for years a strong and militant organizing union, is making relatively little progress now. Its membership has dropped more than 300,000. There is growing dissent within its ranks: Members criticize the leadership for failure to fight harder for job security, safety, and other measures that they say are in the best interests of the rank and file.

Meanwhile, the IBT's formerly close ties to the Reagan administration have weakened with Williams's court problems. Although the union is still one of a scant handful, and by far the most important, to have access to the White House, the union now wields little influence there.

Pressures on the union are mounting. While they are not expected to be heavy enough to seriously damage the IBT, the leadership will be affected as long as Williams is distracted by his legal battles.

Strongly centralized in the union president's office under James R. Hoffa, control over the IBT loosened under his successor, Frank Fitzsimmons. Under Williams, IBT leaders in regional districts and large cities are assuming strongly decentralized power.

According to Ken Paff of a Detroit-based dissident group, Teamsters for a Democratic Union, Williams ''hasn't asserted any leadership - he's letting things go.'' Many leaders who oppose the small and ineffective dissident group say much the same thing. They insist that lower-level leaders have moved into the vacuum to keep the IBT as strong and effective as possible in difficult times caused by the recession and government deregulation.

Edward Lawson, an IBT vice-president, says, ''I think Williams has done very well under difficult conditions since his election in June 1981.''

He was elected then enthusiastically, although it was common knowledge in the union that he was under federal indictments for 11 counts of conspiracy to bribe , wire fraud, and interstate travel to promote racketeering. His four co-defendants have connections with IBT.