Here's my starting point on the question of Catholic politicians and abortion, with regard to Joe Biden's candidacy...
Joe Biden’s record is fairly representative of the broad mainstream of U.S. Catholics. Most Catholics find abortion to be a very serious sin, but are simply not convinced that criminalizing women who seek abortions is the response that will change what is too often a “culture of death.” Biden’s record of curtailing abortion-on-demand, and the fact that he has a long record of refusing to march in lockstep with the NARAL folks, will certainly signal to Catholics that he shares their concerns.
Sometimes his language reflects a common confusion, in that he (like many) claims that he cannot completely rule out abortion because he takes the notion that life begins at conception “on faith,” and thus cannot “impose” this on others. The issue of abortion is not a sectarian or faith-based issue. The Catholic Church itself insists that the absolute dignity of the unborn is not a claim of faith (like believing in the resurrection of Christ, or the virginity of Mary); it is a basic moral claim available to anyone who looks at the evidence. Opposition to abortion does not require a “leap of faith,” there is nothing particularly “Catholic” or “religious” about it.
What complicates the issue, and feeds this confusion, is that a few U.S. bishops insist on holding up this single issue, and making it the sole litmus test for something that IS a “faith issue” – the fitness of a Catholic to receive communion, particularly the fitness of those in public life. It is certainly appropriate for bishops, indeed for all Catholics, to participate vigorously in the public discussion about life issues. And, while it is absolutely appropriate for bishops to be concerned about the valid reception of the sacraments, most Catholics – and the vast majority of bishops – deeply do not want the Eucharist to be politicized. My sense is that most Catholics are not particularly swayed by bishops waving and withholding the Eucharist during political campaigns. For some pro-life Catholics, this seems brave and counter-cultural. For many others, including many pro-life Catholics, it seems merely authoritarian – and opportunistic.
Will Biden's position as Obama's running mate re-start the "wafer wars?" Yes. Will Catholics reject him at the polls for this? I don't think so.