The 1845 Texas Constitution apportioned the state's counties into 19 senate districts. One member was elected from each district and certain counties were represented by two senators. Two classes of senators served staggered four-year terms.
Apportionments under the 1845 constitution were based on state-counted enumerations of qualified electors, required to be reported to each regular session of the legislature until 1850 and every eight years thereafter. Therefore, counties were reapportioned to senate districts with regularity until 1860. During this period membership of the senate grew from 19 to 33.
The 1869 Texas Constitution fixed the number of senators at 33, but apportioned counties to only 30 senators elected in three classes to staggered six-year terms. The legislature was required to conduct reapportionments at the first meeting of the legislature following publication of each United States census.
Current law is derived from the 1876 Texas Constitution. Membership of the senate was fixed at 31 members elected to staggered four-year terms. Each county was limited to one senator and the apportionment was based on qualified electors (these last two requirements were removed by constitutional amendment in 2001).
The legislature enacted new senate districts, following publication of the federal decennial census, through 1901. In 1911, the 32nd Legislature, 1st Called Session, passed a senate apportionment bill, which the governor vetoed. Subsequently, senate reapportionment was not addressed until 1921, when it was taken up by the 37th Legislature, 1st Called Session, following the 1920 census.
The legislature did not enact a senate reapportionment bill following the census in 1930 or 1940. An amendment to the constitution, adopted by voters in 1948, created the Legislative Redistricting Board, with the power to reapportion state house and senate districts if the regular session of the legislature failed to do so following the decennial census. Subsequently, the legislature enacted regular reapportionments of the senate in 1951 and 1961.
In 1964, the U.S. Supreme Court in Reynolds v. Sims ruled that the population of state legislative districts should be relatively equal. Under the 1961 reapportionment, Harris County was represented by only one senate district, yet its population was more than eight times larger than the population of the least populous senate district. That same year, in Kilgarlin v. Martin, a federal court held the Texas constitutional provision that restricted each county to one senator unconstitutional. In 1965, the legislature enacted senate districts that divided the large urban counties into multiple single-member districts for the first time.
In 1971, following the failure of the legislature to enact new senate districts after the federal census, the Legislative Redistricting Board (LRB) convened for the first time and ordered new senate districts. After surviving a challenge in federal court, the LRB plan remained in effect through the 1980 elections.