The international convention of the International Longshoremen's and Warehousemen's Union is expected to approve next month a motion to affiliate with the AFL-CIO union federation.

Such a move would represent a major change in direction for the 24,000- member union, which has a history of staunch independence stemming back half a century.The 14.1-million-member AFL-CIO has already agreed to the affiliation, so the vote at the international convention in Vancouver April 11-15 represents the last hurdle. The ILWU's international executive board voted to recommend affiliation earlier this month.

The need for greater unity has grown more pressing, said Jim Herman, president of the ILWU, in a written statement to union members. All of us need all the help and mutual support we can get.

The San Francisco-based ILWU, which represents shoreside workers on the U.S. West Coast, was formed in 1937 in a break with the International Longshoremen's Association. The ILA, based in New York, represents longshoremen on the U.S. East and Gulf Coasts and on the Great Lakes.

The ILWU joined the Congress of Industrial Organizations, or CIO, in 1937, but was expelled along with several other unions in 1950 for supposed communist domination.

A spokesman for the ILWU said the union will be able to join the AFL-CIO, the nation's largest labor organization, with its autonomy and jurisdiction intact. We can go in with theattitude that we can make a real contribution and all be stronger for it, he said.

A source close to the union said the timing reflects increased pressure on the ILWU and organized labor in general in recent years. As unions have seen more competition from non-union operations and other unions, the number of turf battles have increased.

Affiliation with the larger AFL-CIO would provide an umbrella to mediate jurisdictional questions, he said. It would also provide greater bargaining leverage. The move to affiliate is being made now, he said, because the union faces more problems than it has in the past.

Mr. Herman said the affiliation will not be a cure-all for union problems but will give the ILWU more tools with which to work.

Affiliation raises new questions about the relationship between the ILWU and the ILA.

John Bowers, president of the ILA, said he thought the ILWU should reaffiliate with the ILA rather than maintain its independence under the AFL- CIO umbrella.

I'm not happy with it, he said in a telephone interview. I would much rather have them back in the ILA the way it was when they left.

Mr. Bowers said he gets along well with the ILWU and a reunited union would hold a much stronger negotiating position. You'd either have labor peace or a strike on both coasts, he said. He added that he would have to meet with his executive committee on the whole issue.

The ILWU, which had been the Pacific Coast district of the ILA, emerged under the leadership of Harry L. Bridges. Mr. Bridges was a former Australian merchant seamen who had come ashore around 1920 to work on the docks.

In 1934 he made his name in a San Francisco general strike that shut down most of the city and resulted in several union deaths at the hands of the police.

The ILWU split with the ILA in 1937 because it felt it wasn't getting good representation from the West Coast ILA officials.

On June 21, 1938, the National Labor Relations Board ruled that the entire Pacific Coast could be represented by one bargaining unit, the ILWU.

Over the next few decades, the U.S. government tried to deport Mr. Bridges several times, an effort made easier by his Australian nationality and his often radical political views, but it was not successful.

Although no proof ever emerged that Mr. Bridges was a card-carrying member of the American Communist Party, as some charged, he did become a conspicuous target as the anti-Communist fervor swept the nation in the early 1950s.

On June 3, 1949, the Pacific Maritime Association was formed from two management groups and became the management negotiating body on the West Coast.

One of Mr. Bridges' crowning achievements was the signing, in October 1960, of the mechanization and modernization agreement paving the way for containerization. This pact with the PMA basically accepted containerization in return for share of profits.

The ILWU was never tainted with charges of ties to organized crime as has been the case on the East Coast with the ILA. It is generally credited with maintaining better management relations over the last several decades.