Just where is it to be found in the Catechism that bishops may NOT give specific instructions as to whom Catholics may vote?

Though the authors are absolutely not friendly to Catholicism and pro-life philosophies, "Colonialism, Catholicism, and Contraception" (Annette B. Ramirez de Arellano and Conrad Seipp, University of North Carolina Press, 1983) presents a deeply disturbing history of how the people of Puerto Rico were exploited for "contraceptive" testing. It also provides some fascinating background, related to instructions from bishops on voting:

• In 1960, the authors indicate that reigning political powers were strongly supportive of population control and "contraceptives": 

  "a group of Catholic laymen organized the Christian Action Party (CAP)....The appeal of the new party was based almost exclusively on its support of religious instruction for public school pupils and its objection to existing legislation on birth control and sterilization....
  
  "Bishops James P. Davis of San Juan and James E. McManus of Ponce issued a pastoral letter describing the CAP as `the answer to the intolerable attitude' of the island's established political parties....
  
  "political controversy was heightened when Francis Cardinal Spellman of New York visited Puerto Rico....Spellman was asked to comment on the bishops' involvement with the CAP. Spellman's diplomatic reply was, `I keep out of politics. It is outside my competence and will,' thereby implying that Davis and McManus should do the same.
  
  "Scarcely a week later, ...[Bishops Davis and McManus] issued a pastoral letter prohibiting Catholics of Puerto Rico from voting for [CAP's opposition]....John F. Kennedy, who aspired to be his country's first Catholic president, quickly reacted to the pastoral letter, calling the bishops' action `wholly improper'.... the Kennedys attempted to quarantine the Puerto Rican dispute. Subsequent declarations of several distinguished Catholic prelates accomplished [such]" (pp. 150 - 153). 

• By 1965, Bishops Davis and McManus were gone, as was their style. Henceforth, "the church was vigilant but not vocal" (p. 160).

Scarcely more than a half century ago on U.S. soil, Bishops Davis and McManus were abundantly clear that Catholics could NOT vote for specific candidates who promoted contraceptives and sterilization.  They told their people exactly for whom they could NOT vote!

 Though there may be IRS difficulties with the style of Bishops Davis and McManus, where does the Catechism say that this cannot be done?  Though it's often presented as a given that it cannot be done, I find nothing in the Catechism to even suggest such.

If it's there in the Catechsim (or Canon Law), please let me know.