Roman Catholic on politics

Although Roman Catholic church generally holds political opinions opposite to mine, I don't always have problem with individual Catholic church goers. My mom is the best example. Somehow I feel like the Pope is trying to make me have more problems with the Catholic individuals.

Pope Benedict XVI began his first papal trip to Latin America stressing church opposition to abortion Wednesday, suggesting that Catholic politicians in Mexico had excommunicated themselves by legalizing abortion in that nation's capital.

Benedict, who will inaugurate an important regional bishops' conference during his trip, also spoke strongly against abortion during his first speech in Brazil. Speaking in Portuguese, he said he's certain that the bishops will reinforce "the promotion of respect for life from the moment of conception until natural death as an integral requirement of human nature."

"Do you agree with the excommunications given to legislators in Mexico City on the question?" a reporter asked.

"Yes," Benedict replied. "The excommunication was not something arbitrary. It is part of the (canon law) code. It is based simply on the principle that the killing of an innocent human child is incompatible with going in Communion with the body of Christ. Thus, they (the bishops) didn't do anything new or anything surprising. Or arbitrary."

My first question is where in the Bible says life starts at fetus? As John Paul Stevens wrote in Roe v. Wade, in no where US Constitution recognizes an unborn fetus as a person with Constitutional rights. Then, even if they believe abortion is not good according to Bible, where is the logical linkage for them to make it illegal for the general public, who may or may not have Catholic faith? I surely have no problem if they give excommunications to those had an abortion, but legalizing it? Should those legalizing saying "Oh my God" be given excommunication too? I thought it's called Third Commandment.