Apparently vote trading is illegal in California when done by elected officials:
Recent amendments to the Penal Code extended the prohibition to local officials:
Any member of the legislative body of a city, county, city and county, school district, or other special district, who … gives, or offers or promises to give, any official vote in consideration that … another member of the legislative body … shall give this vote either upon the same or another question, is punishable by imprisonment in the state prison for two, three, or four years and … by a restitution fine of not less than $2,000 or not more than $10,000.
To underscore the seriousness of the offense, vote-trading (like other forms of bribery and crimes against the legislative power) also subjects an official to forfeiture of office and forever being disqualified from holding office.
Which other US states have a similar prohibition on vote trading?
California penal code §86 in more detail:
Every Member of either house of the Legislature, or any member of the legislative body of a city, county, city and county, school district, or other special district, who asks, receives, or agrees to receive, any bribe, upon any understanding that his or her official vote, opinion, judgment, or action shall be influenced thereby, or shall give, in any particular manner, or upon any particular side of any question or matter upon which he or she may be required to act in his or her official capacity, or gives, or offers or promises to give, any official vote in consideration that another Member of the Legislature, or another member of the legislative body of a city, county, city and county, school district, or other special district shall give this vote either upon the same or another question, is punishable by imprisonment in the state prison for two, three, or four years and, in cases in which no bribe has been actually received, by a restitution fine of not less than four thousand dollars ($4,000) or not more than twenty thousand dollars ($20,000) or, in cases in which a bribe was actually received, by a restitution fine of at least the actual amount of the bribe received or four thousand dollars ($4,000), whichever is greater, or any larger amount of not more than double the amount of any bribe received or twenty thousand dollars ($20,000), whichever is greater.
In imposing a fine under this section, the court shall consider the defendant’s ability to pay the fine.
(Amended by Stats. 2014, Ch. 881, Sec. 2. (AB 1666) Effective January 1, 2015.)