Mayor John Lindsay of New York had ordered City Hall flags flown at half-mast to mourn the Kent State students. The hard hats invaded City Hall and raised the flags.
photo: Hard Hats on the Cabinet Table, White House. May 26, 1970. Nixon Library / Wikipedia
Part of the point of the "Hard Hat Riots," which were more like an organized assault than a riot, was to demonstrate that labor -- the common American citizen -- supported President Nixon and the war in Vietnam. In fact, workers were divided on the war. The AFL-CIO took a strongly anti-communist position in support of the war, whereas many leaders and many in the rank and file opposed the war.
Brennan was rewarded after the 1972 election by being appointed Secretary of Labor, a post he retained in the Ford administration.
This event became part of the staging of the narrative, which by now is quite familiar, of the decent hard-working American versus the malingering and disloyal cultural and intellectual elite. It is the same theme as that of the "silent majority" appealed to in Nixon's Vietnam speech of November 1969.
Wikipedia, "Hard Hat Riots."
Thomas W. Benson, Posters for Peace: Visual Rhetoric and Civic Action (University Park: Penn State University Press, 2015).