As is well known, Biden is solidly pro-choice, which puts him at odds with official Catholic teaching on abortion. In that regard, he is akin to Massachusetts Senator John Kerry, another pro-choice Catholic, whose nomination for president four years ago unleashed what came to be known colloquially as the “Wafer Wars.”

In brief, the issue is whether a Catholic politician with a clear record of opposition to church teaching on “life issues,” with abortion holding pride of place, ought to be denied Communion – the consecrated bread and wine which Catholics believe to be the body and blood of Jesus Christ.

In principle, it’s a question that doesn’t just apply to presidential candidates but to public officials at all levels. In reality, however, the massive visibility of a presidential campaign raises the stakes in a unique fashion.

While the American bishops are compactly pro-life, they’re divided as to whether to rebuff pro-choice politicians at the Communion line. An influential minority of bishops believes that publicly denying Communion is essential to defending church teaching, while a majority believes that Communion should never be politicized, even in the service of a good cause.

The latter group takes the position that during Mass, the priest or other minister of Communion is not in a position to judge the consciences of people who come forward. The burden is therefore on individual Catholics themselves to make sure they’re in what the church has traditionally referred to as a “state of grace" before they take Communion. This was the line quietly taken during Pope Benedict XVI’s recent visit to the United States, when pro-choice Catholic politicians such as House Speaker Nancy Pelosi and former New York Mayor Rudy Giuliani received Communion at papal Masses (though not from the pope himself.)

In 2004, following agonizing debate, the U.S. bishops decided that they could not arrive at a uniform national stand on this question, and therefore it would be up to each bishop to set policy in his diocese.

That’s fully in keeping with Catholic theology, which regards each bishop as the supreme authority in his diocese, answerable only to the pope. Yet it also means that a national candidate could be treated differently depending upon which diocese he or she happens to be in on any given Sunday. Such disparities in turn fuel perceptions of division in the church, which is something the bishops always abhor.

Heading into the 2008 campaign season, there weren’t any pro-choice Catholics in the top tier of Democratic candidates, but there certainly was on the Republican side in the form of Rudy Giuliani. Many bishops were, therefore, privately relieved when Giuliani’s campaign imploded, because it at least seemed to mean that the “Wafer Wars” would not be back in the headlines – except, perhaps, with much less intensity on the state and local levels.

Biden’s nomination implies that sense of relief may have been premature.

For the record, Catholic sources in Delaware tell me that Biden is a regular communicant at two local parishes, St. Joseph on the Brandywine in Greenville and St. Patrick. (The latter offers a Sunday evening Mass, which Biden sometimes attends when he’s busy doing Sunday morning talk shows.) He attended Archmere Academy in Claymont, Delaware, sponsored by the Norbertines, and has been active in supporting the school over the years.

Bishop Michael Saltarelli of Wilmington is part of that majority of the American bishops reluctant to turn Communion into a political battleground, and thus has never issued instructions to turn away Biden or other pro-choice politicians. Saltarelli recently retired, and his replacement, Bishop William Francis Malooly, will be installed in early September. To date there’s no indication that Malooly, a native of Baltimore, intends to reverse Saltarelli’s approach.

If nothing else, Biden’s nomination means that advance teams for the Democrats have one other complexity to factor into scheduling campaign stops for their VP nominee. If Biden is planning to be in a given town on Sunday, they’re going to have to put out quiet feelers to make sure that if he shows up at a local parish for Mass, he’s not going to be publicly snubbed for Communion. (Kerry advance teams were forced to do the same thing in ’04.)

Though it’s unlikely he knew it at the time, Biden may have caught a small break back in June when Archbishop Raymond Burke of St. Louis was named to a new position in the Vatican. Missouri is likely to be a battleground state, which means that Biden may need to spend some time there, and Burke was among the most vocal proponents among the American bishops of a tough position on denial of Communion to pro-choice candidates.

Burke’s replacement may not be named before Election Day, so a Biden visit to St. Louis temporarily may be a slightly less dicey proposition.

It’s difficult to say what political fallout all this may have.

Historically, vice-presidential nominees haven’t had a great deal of impact on how elections turn out, which suggests that whatever turbulence Biden may encounter on the campaign trail – for this or any other reason – may not mean much. Further, American Catholics are hardly monolithic, and their votes tend to reflect the overall national population rather than being driven by marching orders from the bishops or from Rome.

Nonetheless, if a few heavily Catholic counties in Ohio had gone the other way in ’04, pundits might well be handicapping the re-election of President John Kerry today. Republicans made aggressive efforts to court Catholics in 2004, and Bush captured a narrow majority of Catholic votes – a majority that was even higher among Catholics who attend Mass at least once a week.

The 67 million Catholics in the United States represent one-quarter of the national population, and they’re over-represented in “Swing States” such as Ohio, Pennsylvania, Michigan, and Texas. If something nudges even one or two percent of Catholics one way or the other, that’s potentially a pool of almost a million votes – enough, perhaps, to make a difference in a tight race.

For now, only one thing seems reasonably certain: America’s Catholic bishops found themselves on Saturday suffering anew from a familiar headache.

Of course, if Obama’s narrow lead in most polls holds up through November, that headache may go on for a long time. The bishops' quandary over Kerry more or less died with his campaign, but if Biden is actually sworn in as vice president next January, the bishops could face at least four years of awkward debates about to what extent they should shun the second-highest office holder in the country.

That’s probably a prospect few of them would relish, wherever they may stand on other matters.