Well, he was endorsed by the Communist-controlled United Electrical, Radio, and Machine Workers of America union in his 1948 senate race against Robert LaFollette Jr.
First of all, the race was in 1946, not 1948. Second, what the pro-Communist unions in the left wing of the CIO (who controlled the *Wisconsin CIO News*) did was not to "endorse" McCarthy over Lafollette (or vice-versa) but to urge their members to vote in the Democratic primary for McMurray, the Democratic candidate (who was a friend of Henry Wallace's)--even though McMurray was unopposed! This of course indirectly helped McCarthy, but one should not exaggerate this. As David M. Oshinsky pointed out in *Senator Joseph McCarthy and the American Labor Movement* (University of Missouri Press 1976), "The volume of the publicity used by the Wisconsin Communists against La Follette in 1946 has been greatly exaggerated. Indeed, during the last three months of the primary campaign, the *Wisconsin CIO News* made only two references to La Follette--both derogatory." (p. 29) Oshinsky also notes that a survey of the readers of another pro-Communist union newspaper (the United Electrical Workers' *UE News*) showed that very few of them believed the newspaper was telling the truth about US foreign policy or the Soviet Union, and suggests that *Wisconsin CIO News* readers were just as unlikely to follow that newspaper blindly on such issues.
However, there is another way in which the Communists may have helped to defeat La Follette. In the Fourth Congressional District (the mostly working-class and heavily Polish-American south side of Milwaukee) the state CIO backed Edmund Bobrowicz, a staff member of the Fur and Leather Workers Union (a union which was unusual even within the left wing of the CIO in being led by an open Communist, Ben Gold) against incumbent Democratic Congressman Thaddeus Wasielewski, who had been critical of the USSR and the Soviet-dominated government of postwar Poland. According to Robert W. Ozanne, *The Labor Movement in Wisconsin: A History* (Madison: State Historical Society of Wisconsin 1984), pp.144-5, "By endorsing Bobrowicz in the Democratic primary the state CIO was perhaps attempting to kill two anti-Soviet birds with one stone. In Wisconsin's primary elections, voters had to choose a party ballot, and to vote for Bobrowicz meant choosing the Democratic party primary ballot. Having done so, the voter could not support La Follette, who was on the Republican party primary ballot. To the extent that the state CIO's aggressive campaign for Bobrowicz helped to defeat Wasielewski, it also helped to defeat La Follette. Bobrowicz's 12,000 votes were more than enough to have defeated La Follette, who lost to McCarthy by only 5,400 votes statewide." (Bobrowicz went on to lose to a Republican in this heavily Democratic district in November.)
Of course, one cannot be sure that all the Bobrowicz voters would have supported La Follette if Bobrowicz had not been running. And indeed, a problem with the argument that the Communists helped elect McCarthy, is that despite La Follette's good labor record, even most *anti-* Communist unions would not support him. Even the AFL, which nobody could accuse of being Communist-dominated, was lukewarm about La Follette. Organized labor had by this time pretty much committed itself to the Demcorats.