When John F. Kennedy (search) ran for president 44 years ago, his faith became a major campaign issue that energized support from fellow Roman Catholics but cost Protestant votes.

Now the Democratic front-runner is another Catholic senator from Massachusetts. But for John Kerry (search), Protestant-Catholic antagonism is negligible; instead, he faces criticism from conservatives in his own denomination who are upset about his stands on abortion and same-sex unions.

The Vatican raised the issue with a decree last year that said Catholic politicians have a duty to uphold the church's "nonnegotiable ethical principles" — specifically mentioning opposition to both legalized abortion and recognition for same-sex couples.

Several U.S. bishops have since made similar statements.

One was Archbishop Raymond Burke (search), who told the St. Louis Post-Dispatch days before the Missouri primary that he would deny Holy Communion to Kerry because he supports abortion rights. Kerry's own archbishop, Sean O'Malley of Boston, has endorsed the principle without naming the senator.

Kerry opposes gay marriage but favors same-sex civil unions. He was among 14 senators who voted against the Defense of Marriage Act, which defines marriage as a heterosexual union for federal purposes, and lets states refuse recognition of gay marriages conducted elsewhere.

Kerry addressed the situation last summer when a second Vatican statement said it's "gravely immoral" for Catholic politicians not to oppose legalization of same-sex couples.

A Catholic who was divorced, remarried and sought a church annulment of his first marriage, Kerry said that "I believe in the church and I care about it enormously. But I think that it's important to not have the church instructing politicians. That is an inappropriate crossing of the line in America."

Similarly, he says he agrees with his church on abortion as a matter of faith but doesn't think he should legislate personal beliefs.

In 1960, candidate Kennedy told a clergy meeting much the same in order to assuage Protestant anxieties.

After Kennedy broke the religious barrier in 1960, three Catholics were nominated for vice president in succession: Republican William Miller (1964), Democrat Edmund Muskie (1968) and Democrat Sargent Shriver (1972).

But the following year, the Supreme Court reshaped Catholic politics by legalizing abortion. Since then, the only Catholic on a major party ticket was Democrat Geraldine Ferraro (1984), who, like Kerry, faced Catholic attacks on abortion.

Anna Greenberg, a consultant for Democrats and abortion rights groups who specializes in religion, doubts bishops' complaints will harm Kerry this year because today's Catholics are "so assimilated and so similar to other voters."

Catholics "tend not to listen to the church on issues like abortion," she said, noting those strongly aligned with the church would back President Bush anyway.

Indeed, University of Notre Dame political scientist David Leege said, "the change in the political and church landscapes has been enormous in the decades between Kennedy and Kerry." Catholics are part of mainstream America and the bishops' moral influence has waned significantly in the last two years because of the clergy sex abuse crisis, he said.

Philip Lawler, the Massachusetts-based editor of catholicworldnews.com, backs the bishops' strong stands on abortion but thinks this could provoke a voter backlash that would help Kerry.

The conservative editor of Crisis magazine, Deal Hudson, sees it another way. While he acknowledges that many Catholic voters aren't affected by the bishops' pronouncements, he thinks the church's stance could be a critical swing factor in a tight race between Kerry and Bush.

Hudson sees this question facing bishops: Will they keep Kerry from using his Catholicism to political advantage by letting him campaign at church colleges, parochial schools and hospitals?

And, he wonders, will liberal "priests and nuns surround him in photo ops?"