Today, many politicians who call themselves Catholic are pro-choice when it comes to abortion. Are these politicians committing a sin by not forcefully condemning abortion? And why do the Catholic bishops not forcefully discipline them?

To be supportive of the so‐called “right” to abortion is indeed sinful and erroneous. Further, to vote to fund abortions is indeed a grave sin. The degree of culpability will be based on how directly the legislator votes for abortion. To go directly to fund abortions is clearly a grave evil. Sometimes however, abortion funding is tucked inside omnibus bills. And thus the culpability of the politician involves a more indirect cooperation with evil. 
 
Bishops and pastors have a serious obligation to warn those who serve in public life against directly supporting, and especially funding, abortion. How this is best done is a matter of tactics and prudence. 
 
Consider for example that a man has some obligation to protect his family from home invasion. Theoretically, any number of alternatives might be possible for him. Perhaps he might booby‐trap his property with lethal weapons that would instantly kill any trespasser. On the other hand, he might reasonably conclude that such a method would also endanger others whom might happen upon his property. Thus he might use lesser means, such as alarms, extra locks, and warning signs. 
 
Which methods are employed, might vary from place to place. If a man lives in a country where civil wars are raging, he might use more severe methods. On the other hand, if a man lives where civil law is generally in place, he may feel it is reasonable to use lesser methods. 
 
It is the same with the bishops on how best to deal with dissenting and wayward Catholic politicians. In some cases, public disciplining and refusal of communion may make some sense. But in other settings, many bishops have concluded such measures might make martyrs out of such politicians. Perhaps the bishop will prefer to privately warn a pro‐choice politician that they will have to answer to God. 
 
Many, who are quick to critique the bishops, ought to recall that matters such as these require careful prudence. Many who demand significantly punitive measures do not themselves take this approach in dealing with their own families. The Lord himself warns that, sometimes in our zeal to pull up weeds, we end up harming the wheat. 
 
Reasonable people differ on how to handle matters of prudential judgment. Indeed, individual bishops vary in their approaches. This is the nature of such prudential judgments. Catholics do well to pray for priests and bishops, who have obligations to correct, but must do so in ways that do not cause more harm than good. Pray!