Stephen Ingersoll Zetterberg (October 2, 1916 – January 30, 2009) was an American attorney and Democratic activist. Zetterberg was best known for being defeated by Congressman Richard Nixon in the 1948 Democratic primary, as Nixon, having no Republican primary opposition, entered the Democratic primary to ensure he would have no Democratic opponent in the general election.
Zetterberg was born on October 2, 1916, in Galesburg, Illinois. He grew up in New Castle, Indiana, moving to Claremont, California as a teenager. He went to college locally at Pomona College, serving in the United States Coast Guard during World War II. After the war ended, he worked as a congressional staffer for United States Senator Scott W. Lucas (D-Ill.).
In 1946, Zetterberg returned to Claremont to practice law, and involved himself in local politics. Elected chairman of a local committee seeking to manage growth in the area, Zetterberg urged former congressman Jerry Voorhis, defeated by Nixon in 1946, to run again in 1948. Voorhis declined, citing health reasons, and Zetterberg decided to run in the Democratic primary instead. To attract attention to his campaign, he organized square dances. Nixon refused to debate Zetterberg.
Zetterberg filed for the race on March 23, 1948, just over two months before the June 1 primary, but his commitments as an attorney kept him busy until mid-April. He did not start to campaign even then, but waited until a month before the primary. Zetterberg was underfinanced in the campaign, spending less than $2,000.
At the time, California law permitted candidates to run in both major parties' primaries, a practice known as "cross-filing." In an attempt to assure himself of no major party opposition on the general election ballot, Nixon, who was facing no Republican opposition, also entered the Democratic primary.. Under instructions from the Nixon campaign, a group called "Democrats for Nixon" sent out postcards addressed to "Fellow Democrats". Under California law at the time, the incumbent was listed first on the ballot, and was identified as such. According to Zetterberg in a 1972 interview, some voters showed him their sample ballots, convinced Nixon was the Democratic incumbent.