LI Legislature of the Mexican Congress

{{Infobox legislative term

|name=51st Congress
(LI Legislatura)

|before=50th

|after=52nd

|election=1 July 1979

|image=

|body=Congress of the Union

| meeting_place =

| term_start = {{Start date|1979|09|01|df=y}}

| term_end = {{End date|1982|08|31|df=y}}

| chamber1 = Senate of the Republic

| chamber1_image =

| chamber1_image_size =

| chamber1_alt =

| membership1 = 64

| control1 =

| chamber1_leader1_type = President

| chamber1_leader1 =

| chamber2 = Chamber of Deputies

| chamber2_image =

| chamber2_image_size =

| chamber2_alt =

| membership2 = 400

| control2 =

| chamber2_leader1_type = President

| chamber2_leader1 =

}}

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 sat from 1 September 1979 to 31 August 1982.

The session's senators had been elected in the 1976 general election while the deputies were elected in the 1979 mid-term election.

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. The session's members included the first deputies from the Mexican Communist Party (PCM), the Workers' Socialist Party (PST) and the Mexican Democratic Party (PDM).{{cite web |title=La Reforma política de 1977 |url=https://www.diputados.gob.mx/museo/s_nues11.htm |website=Cámara de Diputados |publisher=Government of Mexico |access-date=7 July 2024}}{{cite web |title=Comunistas en el Legislativo |url=https://memoricamexico.gob.mx/es/memorica/Temas?ctId=3&cId=MTc=&cd=true |website=Memórica |publisher=Government of Mexico |access-date=7 July 2024}} The reforms also increased the number of deputies: 300 from single-member constituencies and 100 from regional lists.{{cite journal |title=Evolución territorial de los distritos electorales federales uninominales, 1977–2010 |last1=Baños Martínez |first1=Marco Antonio |last2=Palacios Mora |first2=Celia |trans-title=Territorial evolution of the federal uninominal electoral districts, 1977–2010 |journal=Investigaciones Geográficas |publisher=Instituto de Geografía, UNAM |place=Mexico City |date=2014 |issue=84 |page=92 |doi=10.14350/rig.34063 |url=https://www.investigacionesgeograficas.unam.mx/index.php/rig/article/view/34063/39223 |access-date=3 July 2024}}