Senators in Mexico cannot immediately run for re-election and thus, the Senate is completely renewed every six years
“The Senate of the Republic, (Spanish: Senado de la República) constitutionally Chamber of Senators of the Honorable Congress of the Union (Spanish: Cámara de Senadores del H. Congreso de la Unión), is the upper house of Mexico’s bicameral Congress.
After a series of reforms during the 1990s, it is now made up of 128 senators:
Two for each of the 31 states and two for the Federal District elected under the principle of relative majority;
One for each of the 31 states and one for the Federal District, assigned under the principle of first minority (i.e. awarded to the party who had won the second highest number of votes within the state or Federal District);
Thirty-two national senators-at-large, divided among the parties in proportion to their share of the national vote.
In a senatorial race, each party nominates two candidates who run and are elected together by direct vote. The party of the two candidates that won the second highest vote within the state or the Federal District then assigns a senator to occupy the third seat (first minority seat), according to the list of candidates that the party registered with the Federal Electoral Institute (IFE).
Senators serve six-year terms, running concurrently with the President of Mexico. Special elections are rare, as substitutes are chosen at every election.
The Senate is completely renewed every six years, since senators are barred from immediate reelection.”