The National Labor Relations Board, which (almost) never saw an employer anti-union tactic it (really) didn't like, rules that pro-union sentiments among managers make an election invalid:

Even employers who force employees to attend anti-union meetings are acting lawfully. Yet the Bush-appointed majority of the Board recently ruled that pro-union conduct by a supervisor was objectionable, coercive, and grounds for overturning a five-year-old union election victory. In 1998, the employees of Harborside Healthcare, a nursing home in Beachwood, OH, sought to form a union with the Service Employees International Union (SEIU). Robin Thomas was one of the Harborside employees who participated in the union campaign. She told her co-workers about the benefits of joining a union and asked them to sign cards authorizing SEIU as their representative in collective bargaining. Despite Harborside's anti-union stance, the workers voted 49 to 36 in favor of union representation on October 1, 1998. The company filed to overturn the results of the election with the NLRB, charging that Thomas was a supervisor and that her pro-union conduct during the organizing campaign coerced employees into voting for union representation. In a unanimous decision, the Board denied Harborside's appeal and certified the results of the election in 1999. The Board members ruled that Thomas was a supervisor under the law, but that her conduct was not objectionable because she did not explicitly threaten or promise benefits to her co-workers. Harborside appealed the ruling. Rather than ruling on the case, a federal appeals court remanded the case back to the Board for further clarification in 2000. On December 8, 2004 - after sitting on the case for four years - the board finally issued a 3 to 2 decision overturning the results of the election. In its ruling, the Board majority determined that the pro-union position of one low-level supervisor was enough to taint the election, despite the fact Harborside ran an anti-union campaign. Among the conduct they found objectionable were Thomas' statements describing the benefits of forming a union for job security and the solicitation of signatures for a pro-union petition, the latter being an explicit reversal of a previous Board decision. The dissenting members of the Board charged the majority with creating an "arbitrary double standard" in their treatment of pro vs. anti-union conduct. The Board has long allowed employers to force employees to attend 'captive audience' meetings where they are often deluged with anti-union propaganda. Yet the majority found it objectionable for Thomas to simply urge her co-workers to attend union meetings. In their dissent, the minority argued, "Contrary to the majority's view, the law does not apply more harshly to pro-union supervisors…than to anti-union supervisors…" If this new standard were applied to anti-union conduct—the Board would ban much of what has become standard anti-union tactics used by employers to crush organizing efforts.