The LI Legislature of the Congress of the Union (LI Legislatura del Congreso de la Unión) was the 51st session of the Congress of Mexico.
It marked a watershed in that it was the first congress elected after the 1977 political reforms, under which several parties on the left and the right obtained official registration and access to Congress by opposition parties (i.e. those other than the hegemonic Institutional Revolutionary Party) was made much more viable.
[1][2] The reforms also increased the number of deputies: 300 from single-member constituencies and 100 from regional lists.
[3] The 1976 general election had elected two senators for each state and the Federal District – a total of 64 – to serve six-year terms.
The 51st Congress was elected in the July 1979 mid-term election following the 1977 political reforms: the number of single-member constituencies was increased to 300 (from 196 in the 50th Congress), the proportional representation system was expanded to 100 seats (compared to 41 in the 50th Congress), and three parties obtained their first deputies: the Mexican Communist Party (PCM), the Workers' Socialist Party (PST) and the Mexican Democratic Party (PDM).