Moving to end a 13-year rift in the house of Labor, the executive board of the United Auto Workers voted unanimously yesterday to reaffiliate with the AFL-CIO and promptly vowed a consolidated attack against President Reagan's economic policies.

UAW President Douglas Fraser said in Detroit that his union, the nation's third largest, will rejoin the federation at the national level, perhaps as soon as July. State and local UAW units will be free to decide whether to reaffiliate with their own AFL-CIO counterparts, but the national merger was settled yesterday.

One of the principal reasons for the step, a spokesman acknowledged, was to give the labor movement more clout in what it regards as parlous times. The addition of the UAW's 1.2 million U.S. members will give the AFL-CIO close to 15 million members.

Fraser said the move was being made "in one of the most difficult periods working people have ever faced."

But he made plain to reporters that his desire for unity had its limits. He said he personally had "absolutely no desire" to see the Teamsters Union -- the nation's biggest -- back in the fold.

AFL-CIO officials welcomed the auto workers' decision and said a quick poll of its 35-member Executive Council to ratify the move would be conducted as soon as formal notification from Fraser arrives.

"Everybody's very, very pleased and happy," AFL-CIO spokesman Saul Miller said of the drawn-out process that was launched at a UAW board meeting last December.

Lane Kirkland, who succeeded George Meany as head of the federation in November 1979, had made the reaffiliation one of his major goals. When Kirkland took office, Miller recalled, "he said 'all sinners belong in church and all unions belong in the AFL-CIO.'"

Fraser said many details remain to be worked out, "but we want to move quickly to solidify joint efforts on a range of economic and political initiatives."

Among these, he said, would be concentrated opposition to President Reagan's proposed cuts in Social Security benefits and a continuing campaign to persuade both the White House and the Federal Reserve Board "to lower interest rates and abandon recessionary economic policies."

In addition, Fraser said, "we'll continue our efforts with the AFL-CIO to save important governmental programs that are being callously slashed in Washington without a sense of the havoc those cuts will mean, not only for the disadvantaged, but also for working and middle-class America."

The UAW left the federation in 1968, partly because of personality differences between Meany and then UAW president Walter Reuther, partly because of differences over issues ranging from foreign policy to organizing efforts. Working-class America has changed considerably since then. At the time of the split, the UAW was by far the biggest union in the AFL-CIO, but now unions of service employes, such as the 1,076,000-member United Food and Commercial Workers, are moving up rapidly.

The UAW will begin paying $2.5 million a year to the federation from its international coffers without any change in the union's dues structure. The assessment in 1968 was fixed at 10 cents a member each month. Now it is 19 cents.

UAW spokesman David Mitchell said not all differences between his union and the AFL-CIO would disappear, but he said there is a strong feeling in the UAW that "if there is a central labor body in existence, we ought to be a part of it and if there is not agreement with some of the policies, we ought to try to change them. On most issues we are in complete agreement."

The move will leave the Teamsters, the National Education Association and the United Mine Workers as the three biggest outside the federation.

Fraser said he was not eager to see the Teamsters come back "because of their bad reputation which, I think, is deserved in many places." He said some Teamsters locals are above reproach, but "you just look at the number of indictments against [Teamster] officials" around the country.

The Teamsters' newly installed president, Roy Lee Williams, was indicted last month on charges of conspiring to bribe Sen. Howard W. Cannon (D-Nev.).