He’s not on the activist level like those other two. Does he even publicly endorse candidates?
Are you joking? I usually don't like cut/paste wiki, but his page even has a section on his political activity, as I have referenced some below:
Politics
Spielberg usually supports U.S. Democratic Party candidates. He has donated over $800,000 to the Democratic party and its nominees. He has been a close friend of former President Bill Clinton and worked with the President for the USA Millennium celebrations. He directed an 18-minute film for the project, scored by John Williams and entitled The American Journey. It was shown at America's Millennium Gala on December 31, 1999, in the National Mall at the Reflecting Pool at the base of the Lincoln Memorial in Washington, D.C.[97]
Spielberg resigned as a member of the national advisory board of the Boy Scouts of America in 2001 because of his disapproval of the organization's anti-homosexuality stance.[98][99]
On February 20, 2007, Spielberg, Katzenberg, and David Geffen invited Democrats to a fundraiser for Barack Obama.[102]
However, on June 14, 2007, Spielberg endorsed Hillary Rodham Clinton (D-NY) for President. While Geffen and Katzenberg supported Obama, Spielberg was always a supporter of Hillary Clinton. However Spielberg directed a video for Obama at the DNC in August 2008 and attended Obama's inauguration.
In September 2008, Spielberg and his wife offered their support to same-sex marriage, by issuing a statement following their donation of $100,000 to the "No on Proposition 8" campaign fund, a figure equal to the amount of money Brad Pitt donated to the same campaign less than a week prior.[107]