Josh Goodman is a former staff writer for GOVERNING..
E-mail: mailbox@governing.comTwitter: @governing
I was quite surprised to discover only a bare majority of states, 26 of them, generally use special elections to fill state legislative vacancies (the alternative is an appointment process, which typically requires that the seat stay in the same party's hands). I discovered that in the process of reporting a story about how the appointment process in Colorado, combined with other factors, has created a wave of early retirements.
The prevalence of appointments was striking to me in the context of the debate this year over how U.S. Senate vacancies should be filled. For legislative vacancies, Western states are especially likely to use appointments rather than special elections.
The National Conference of State Legislatures was kind enough to send me a chart that describes each state's special elections procedures, which I've tweaked into a simpler version below. There are lots of strange variations on how legislative vacancies are filled.
For example, in North Dakota political parties pick replacement legislators. However, according to NCSL:
If 828 days or more remain until the expiration of the term of office, The qualified electors of a legislative district in which a vacancy in the legislative assembly occurs may petition for a special election to be called by the governor to fill the vacancy.I'm not even going to try to describe all of the different appointment processes states use. Suffice it to say, sometimes the appointments are made by other legislators, the governor, county commissions and political parties. Several states use hybrid systems -- for example, a local political party will submit a list of candidates from which the county commission must pick.
Here's very basic list of how each state fills legislative vacancies.
Special elections:
Alabama
Arkansas
California
Connecticut
Delaware
Florida
Georgia
Indiana
Iowa
Kentucky
Louisiana
Maine
Massachusetts
Michigan
Minnesota
Mississippi
Missouri
New Hampshire
New York
Oklahoma
Pennsylvania
Rhode Island
South Carolina
Texas
Virginia
Wisconsin
Usually appointments, but with special elections in certain circumstances:
Montana
Hawaii
Tennessee
Appointments:
Arizona
Nevada
New Mexico
Washington
Wyoming
Alaska
Idaho
Kansas
Maryland
Nebraska
North Carolina
South Dakota
Utah
Vermont
West Virginia
Ohio
Josh Goodman is a former staff writer for GOVERNING..
E-mail: mailbox@governing.com 
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