Bishops and priests can always address issues, but they cannot endorse a candidate. That’s when they lose the tax-exempt status for their church.
....1841 was an election year in the state of New York. Five days before the election, at a Catholic rally at Carroll Hall, [then-bishop John Joseph Hughes] presented his parishioners with a list of the candidates he favored for council and urged them to vote for them....The Constitution gave Hughes the right to advise his parishioners how to vote, but the Protestant establishment was outraged at what they saw as priestly meddling in politics. Leading the attack, James Gordon Bennett, editor of The New York Herald....Hughes's politicking paid off. All but three of the candidates he had supported were elected. In April 1842, the state passed the Maclay bill. By a majority of just one, New York's Senate voted to end religious instruction in New York's public schools....
-- excerpts from Hour Two of the PBS broadcast God In America
The next question then is, why the hell is USCCB so vocal about this particular issue, and on the wrong side for any thinking Christian?