WASHINGTON — One key to Senator Obama’s chances of defeating Senator Clinton in the crucial Pennsylvania primary next month will be his courtship of Catholic voters, a sizable bloc that has loyally supported the former first lady throughout the primary season.

Catholics could make up more than one-third of the vote in the Keystone State on April 22, and Mrs. Clinton’s victories in Ohio, Texas, and Rhode Island on Tuesday made clear that despite a stepped-up effort in recent weeks, Mr. Obama has failed to narrow her edge with that demographic.

Mr. Obama’s campaign has fought aggressively against the perception that he has a “Catholic problem,” but it does not deny the numbers: In state after state that Mrs. Clinton has won, exit polls show that she has captured as much as 65% of the Catholic vote — a percentage that far outstrips her margin of victory. And in several states that Mr. Obama has claimed, including his home state of Illinois, which he won by 22 points, the Catholic vote has gone to Mrs. Clinton.

The Catholic numbers have prompted an intensifying debate about the reason the gap persists, and whether it is due to a specific advantage for Mrs. Clinton or doubts about Mr. Obama, or whether it is merely attributable to overlapping voting constituencies that favor the New York senator.

“The consistent polling has been that the bedrock of her strength is Catholic Democrats,” a Clinton supporter who heads the National Ethnic Democratic Leadership Council, Brian O’Dwyer, said.

Mrs. Clinton’s advantage escapes obvious explanation. She is not Catholic, and while her supporters often point to her involvement in the Northern Ireland peace process as fostering a close connection with Catholics, Mr. Obama has the fervent backing of one of America’s most prominent elected Catholics, Senator Kennedy of Massachusetts, whose brother was the first Catholic president.

Advisers to Mr. Obama have contended that Mrs. Clinton’s edge among Catholics is fueled largely by her well-known support among Hispanic voters, who comprise a large proportion of the Catholic population in large states like California and Texas. The Obama campaign pushed back hard against a Politico article earlier this month that reported on his soft support among Catholics, fearful that the story would become a dominant campaign narrative.

But the March 4 results seem to have put to rest the notion that Mrs. Clinton’s Hispanic base is wholly responsible for her advantage among Catholics. In Ohio, which has a tiny Hispanic population, she won 63% of the vote among Catholics, who comprised one-fifth of all voters in the primary, according to an NBC News exit poll. And in Rhode Island, where white Catholics made up 46% of the primary electorate, Mrs. Clinton took 69% of the demographic. Mr. Obama did win the Catholic vote in Vermont, but his advantage was much thinner than his margin of victory overall.

The percentages bode well for Mrs. Clinton in Pennsylvania, whose large Catholic population is predominantly white.

Looking ahead to next month, Mr. Obama’s campaign is actively reaching out to Catholic voters. The senator in February sent a letter to thousands of nuns across the country, and the campaign was pleased with several “community faith forums” it held in Ohio, including two directed at Catholic voters specifically that featured a former Indiana congressman, Timothy Roemer, as well as Victoria Kennedy, the wife of Senator Kennedy. The campaign plans several more of those events in Pennsylvania, Mr. Obama’s director of religious affairs, Joshua DuBois, said in an interview.

In addition to Mrs. Clinton’s support with Hispanic voters, the Obama campaign attributes her success among Catholics to her longer tenure on the national stage.

Mr. DuBois downplayed worries that Mr. Obama was not connecting with Catholic voters, and although the campaign is 14 months old, he said the candidate’s introduction to Catholics was only beginning. The campaign, he said, would be “aggressively and robustly reaching out to Catholics” in the weeks ahead.

“There’s no concern there, because this introduction is just getting started,” Mr. DuBois said, “and as more and more Catholics get to know who Senator Obama is and what he stands for, we are 100% confident that he will be the candidate for Catholic voters, not just in the primary but on into the general election.”

Mr. DuBois also pointed to states that Mr. Obama has won with sizable Catholic populations, such as Louisiana and Missouri.

Indeed, there is little evidence to suggest that Catholics have supported Mrs. Clinton, who is Methodist, over a specific issue or because they distrust or dislike Mr. Obama. “We’ve not picked up anybody who says, ‘I’m a Catholic, and I’m voting for Hillary,'” a political scientist who directs the Franklin and Marshall Poll in Pennsylvania, G. Terry Madonna, said.

There is also no evidence that false speculation about Mr. Obama being a Muslim — he is a Christian — have had an outsize impact on Catholics, nor is there data indicating that race has been a factor in the Catholic vote.

Mr. Madonna said Mrs. Clinton’s advantage could likely be attributed to a number of factors, in particular the fact that many of the working-class white voters who form the base of her support are Catholic. These Democrats, who tend to vote based on more “bread and butter table talk” issues like health care and the economy, have responded more to Mrs. Clinton’s message than Mr. Obama’s “new kind of politics” brand.

As the Democratic race turns increasingly to the question of electability, the Catholic gap could loom large. Catholics have traditionally been a swing vote, Mr. Madonna said, and many fall into the category of Reagan Democrats, which could be crucial to the chances for victory for either Mr. Obama or Mrs. Clinton over the presumptive Republican nominee, Senator McCain.

“This spells disaster for the Democrats if Obama were the nominee,” Mr. O’Dwyer said, suggesting the senator’s thin ties to the Catholic community would put him at a considerable disadvantage versus Mr. McCain, who he said had a strong appeal among Catholics.

A Democratic consultant who supports Mr. Obama, Michael Tobman, disputed the idea that Catholics would shift en masse to Mr. McCain if Mr. Obama were nominated, but he did acknowledge that the Arizona senator, who identifies himself as a Baptist, could have an advantage, particularly with Irish Catholics.

“For nothing more than his last name, McCain starts with a leg up in the Catholic community,” Mr. Tobman said.