The controversy has been mild so far, but still, every vote counts in a presidential election that is expected to be closely contested.
Evangelical or born-again Christian voters were key to George W. Bush's victories, but so were Roman Catholics, who chose Bush over their fellow Catholic John Kerry in 2004 and over Al Gore in 2000.
The televangelist, San Antonio megachurch leader John Hagee, has referred to the Roman Catholic Church as "the great whore" and called it a "false cult system" and "the apostate church"; the word "apostate" means someone who has forsaken his religion.
He also has linked Adolf Hitler to the Catholic church, suggesting it helped shape his anti-Semitism.
McCain said he does not agree with some of Hagee's past comments. "It's simply not accurate to say that because someone endorses me that I therefore embrace their views," McCain told reporters at a news conference Monday in Phoenix.
Catholic groups are pressuring McCain to go further and reject the endorsement, which he announced at a news conference with Hagee last week. The Democratic National Committee also is publicizing Hagee's views.
"Indeed, for the past few decades, he has waged an unrelenting war against the Catholic Church," said Catholic League President Bill Donohue.
"Senator Obama has repudiated the endorsement of Louis Farrakhan, another bigot," Donohue said. "McCain should follow suit and retract his embrace of Hagee."
He was referring to, who said he would "reject and denounce" any help from Farrakhan when pressed in last week's Democratic presidential debate.
It remains to be seen how much Hagee's views may hurt McCain's standing among Catholics, a group that can hardly be considered monolithic. Though they lean Republican, their views span the political spectrum and split nearly evenly along party lines.
Despite the recent publicity, Hagee is not well-known outside his sphere of influence, which includes a congregation in the tens of thousands and an even wider television audience.
"What he holds about Catholicism in my mind is despicable," said the Rev. James Heft, religion professor at the University of Southern California. "I totally reject Hagee's view of Catholicism, but I don't know how widely known it is."
If Hagee's views become well-known, the endorsement could hurt McCain among some Catholics.
"If you offend even a small percentage, that could make the difference in an election," Donohue said in an interview Sunday.
Democrats are doing their best to keep the fracas alive, with Democratic National Committee Chairman Howard Dean raising it Sunday on CNN's "Late Edition."
"What about a guy who is a vicious anti-Catholic, who is supporting John McCain, and John McCain does not denounce or reject him?" Dean said.
So far, McCain has enjoyed strong support from Catholics, who make up about a quarter of the electorate.
He won far more of the Catholic vote, 47 percent, than any of his Republican rivals thus far, according to exit polling. Mitt Romney won 30 percent andwon 9 percent, doing well among Catholics in states where they did well overall, according to exit surveys in 21 presidential primary states.
McCain has been less popular among evangelical or born-again Christians, which is where Hagee comes in. Huckabee, himself a Baptist minister, courted Hagee last year by delivering a sermon at his church. McCain has lost or split support from those voters and is working to bolster his standing.
And McCain is not guaranteed support among Catholics, even though he opposes abortion and the two Democratic candidates, Obama and, support abortion rights.
While the church places utmost priority on its opposition to abortion rights, U.S. bishops issued voter guidelines last November saying Catholics may vote for someone who favors abortion rights - so long as the voter is not making his or her choice because of the candidate's position on abortion, and if the candidate supports other positions that substantially further the church vision of the common good.
Incidentally, McCain, Obama and Clinton belong to the Protestant faith; McCain was raised Episcopalian but now attends a Baptist church in Arizona.
McCain's response to the controversy has been tepid, Heft said. Following two days of criticism, McCain issued a statement saying only that he doesn't agree with everything Hagee says.
"In no way did I intend for his endorsement to suggest that I in turn agree with all of Pastor Hagee's views, which I obviously do not," McCain said. Before issuing the statement, he told reporters he was "proud" of Hagee's spiritual leadership of his congregation.
The Arizona senator's reaction stands in contrast to President Bush, who specifically apologized to Roman Catholic leaders for "causing needless offense" when he visited Bob Jones University during the 2000 election. The Greenville, S.C., school teaches that Catholicism is a cult.
McCain's reaction also stands in contrast with his own swift and unequivocal denunciation of a radio talk show host who denigrated Obama last week in Cincinnati. McCain immediately apologized and said he repudiated the statements of the radio host, Bill Cunningham.
Of course, there are differences between the two figures. Hagee is a religious leader; Cunningham is a talk show host. Cunningham made his comments at a campaign event; Hagee's intolerant words and views have come outside the presidential campaign.
Regardless, Heft said McCain should be more specific and more emphatic, and soon.
"You don't want to blow it on simple matters that you could correct," Heft said. "He probably would be wiser just to say he rejects his views on Catholics."