Free speech during a union organizing campaign is protected under Section 8(c) of the NLRA which holds that expressing opinions is prohibited only to the extent that it constitutes a threat of reprisal or force, or promise of benefit.

Section 8(c)

Part of the National Labor Relations Act allowing for the expression of views, argument, or opinion by employers, without threat of reprisal or force.

NLRA

The National Labor Relations Act of 1935, a foundational statute in United States labor law which protects the rights of employees to organize, to form, join, or assist labor organizations, to bargain collectively, and to engage in other protected concerted activities.