On June 8, 2010, he lost his third bid for the United States Senate, campaigning once again for the seat held by Democrat Barbara Boxer but losing the Republican nomination to Carly Fiorina.

He served two terms before making an unsuccessful bid for the Republican nomination for the Senate seat being vacated by Alan Cranston.

Despite the considerable disadvantage in voter registration and Democratic attempts to tie him to Speaker Newt Gingrich, Campbell won the December special election easily.

This was not surprising, as Bay Area Republicans tend to be more moderate on social and environmental matters than their counterparts in the rest of California.

[7] Campbell led a group of 17 bipartisan members of Congress who filed a lawsuit against President Clinton in 1999 over his conduct of the war in Kosovo.

Although he touted his service as a moderate Republican representing a strongly Democratic district, he was considered a decided underdog.

[9] In 2008, Campbell wrote in Reason that he would be voting no on Proposition 8, the proposed ballot measure banning same-sex marriage in the state, per his Republican beliefs that "government should be limited.

In the intervening years, he began to assemble the material, out of his professional political experiences, for his book The Separation of Powers in Practice [13] with Stanford University Press.

In September 2004, Campbell was named by California Governor Arnold Schwarzenegger to his newly formed Council of Economic Advisors.

From 2004 to 2005, Campbell took a leave of absence from his Berkeley post to serve as director of the California Department of Finance in the Schwarzenegger administration.

[15] In mid-2008, Campbell joined the Palo Alto office of Los Angeles-based Gibson, Dunn & Crutcher LLP.

In July 2008, Tom Campbell filed the necessary paperwork in order to establish a committee with the intent to raise funds for a prospective race for the Republican nomination to be Governor of California in 2010.

[20] On January 13, 2010, the Wall Street Journal reported that Campbell would run for the United States Senate, instead of for Governor of California.

In the primary on June 8, Campbell finished a distant second to former Hewlett-Packard CEO Carly Fiorina, in a race that also included State Assemblyman Chuck DeVore.

The primary election received national attention, in part due to a campaign ad released by Carly Fiorina, depicting Campbell as a "Demon Sheep".

[21]Campbell also said he could not support the Democratic nominee, Hillary Clinton, either, due to her involvement in the email controversy.

[citation needed] In 2016, Campbell wrote a column for the Orange County Register, which, while not endorsing Gary Johnson for president, suggested libertarian-leaning Republicans should consider him.

[22] Campbell was later one of 30 former Republican members of Congress who wrote an open letter denouncing Trump's candidacy.