An undercurrent of internal dispute at union conventions during the past several weeks has underscored organized labor's frustrations - and its vulnerability.
At the same time, they have pointed up differences between the present union movement, confronted by economic decline, and the vigorous movement of past decades, which made major gains during the Great Depression of the 1930s. Today's bigger, potentially more powerful movement, by contrast, is on the defensive.
Speakers at steel, electrical manufacturing, and other union conventions blamed the economic policies of the Reagan administration, the government's ''toleration'' of high unemployment, and ''disastrous'' American foreign trade policies for such problems as drooping membership and ebbing political clout.
Others took management to task for failing to keep up with the technological breakthroughs being made by competitors abroad. Off the convention floors, some faulted union leaders and ''conservative'' union members for a lack of aggressive trade unionism.
At one convention after another, unions sounded strong rallying cries for the political mobilization of unionists and their allies. The goal: Democratic congressional victories this November and the ouster of Republicans from the White House in 1984.
Delegates listened to and cheered prospective Democractic candidates for the White House - particularly former Vice-President Walter F. Mondale and Sen. Edward M. Kennedy of Massachusetts. But many delegates complained afterward that they had heard only echoes of past anti-Reagan rhetoric. They deplored labor's failure to pioneer a constructive new program for solving the country's problems.
Typical of rhetorical attacks on the administration were those launched at the conventions of the AFL-CIO's International Union of Electrical Workers (IUE) in New York and International Brotherhood of Electrical Workers (IBEW) in Los Angeles. ''He is not a conservative, he is a radical rightist,'' IUE president David Fitzmaurice said of Mr. Reagan. IBEW's president Charles H. Pillard charged that the Reagan administration ''has given new life to the extremists of the right - the very same forces that organized labor threw out of the government 50 years ago.''
Although the Sheet Metal Workers, in New York, joined in the political attacks, that union and others in the building trades, which have convened recently, showed equal concern for such practical matters as the growing use of nonunion labor in construction. Union president Edward J. Carlough called this ''the single most overriding problem facing us.''
Unions of federal employees and postal workers meeting in New York, San Francisco, and Miami Beach were somewhat more temperate in anti-Reagan rhetoric but as determined as other laborites to develop political solidarity in 1982 and 1984 elections.