On May 26, 1970, President Nixon met in Washington with twenty-two New York City labor union leaders, most from unions representing workers in the construction trades (such as painters, electricians, and carpenters). The President thanked the union leaders for their support, and in turn they presented him with a hard hat inscribed "Commander in Chief." Union leader Peter Brennan declared "The hard hat will stand as a symbol, along with our great flag, for freedom and patriotism to our beloved country." After his reelection in 1972, Nixon nominated Brennan to be Secretary of Labor, a post he held from his confirmation in 1973 until 1975.
According to the New York Times, Nixon refused to pose for pictures wearing the hard hat--at least not for the press. This photo comes from the National Archives.
(photo reprinted in William H. Chafe, The Unfinished Journey: America Since World War II)