Members of Congress have become increasingly polarized over such issues as stem cell research, same-sex marriage and abortion, much like their constituents. But the division among policymakers is more pronounced and influenced by religious ideology, according to a study of congressional voting patterns over the last quarter-century.

That Congress mirrors public attitudes on the so-called culture wars came as no surprise to William D'Antonio, a professor at Catholic University who directed the study and has observed the interaction of religion and politics for 50 years.

What did surprise him, D'Antonio said, was the degree to which religious affiliation helped create an ideological divide between Republicans and Democrats that virtually ensures a partisan split on most votes before lawmakers.

Antonio and Steven Tuch, a professor at George Washington University, focused their research on congressional votes related to abortion cast from 1979 to 2003. They checked the religious affiliation of members of Congress against votes, including those on federal funding of abortions and late-term procedures. They then correlated those results with votes on other issues, such as military spending, welfare and tax reform.

What they discovered were trends in voting patterns relating to religious affiliation that went beyond the broadly acknowledged impact of the evangelical Christianity in American politics, which began with the emergence of the Moral Majority in 1980.

The study looks at a period that begins with the 96th Congress and continues through the first half of the current session, the 108th. In 1979, the political parties had not solidified their positions on a woman's right to abortion, affirmed just six years earlier in Roe v. Wade.

Democrats were more likely to support abortion rights and Republicans, antiabortion measures. But members of Congress were more likely to vote on abortion issues based on personal convictions, religious or otherwise -- votes that often conflicted with others in their party.

Today, there appears to be few, if any, disputes within parties over abortion. That is not the case within the general population, where surveys show Americans are divided over abortion, but in nuanced ways.

About 25 percent of Americans support a woman's right to abortion under any circumstances, and about 20 percent oppose abortion for any reason, according to Gallup and other research organizations. Those in between approve of abortion if the woman's health is at risk, in cases of incest or rape, or if the fetus appears deformed.

But on Capitol Hill, ethical nuances have given way to the party line, with Democrats typically favoring access to abortion and Republicans generally rejecting it, according to D'Antonio and Tuch.

"The great middle is not represented," D'Antonio said, referring to the 55 percent of Americans who support a woman's right to choose depending on the circumstance.

So what does religion have to do with this polarization?

James Guth, who since 1995 has examined religious affiliation and voting patterns in the House of Representatives, says a "substantial increase" in the number of evangelical Protestants in Congress, most of them Republicans, has contributed to the moral climate of voting on abortion and other cultural issues.

Estimates on the number of evangelicals in Congress vary, from fewer than 10 percent in the 1970s to more than 25 percent today.

In another important development, Roman Catholic legislators are no longer predominantly aligned with the Democratic Party. Traditional Catholics land on the Republican side and theologically liberal Catholics on the Democratic.

"Religion is much more aligned with partisanship than it was in the past," said Guth, a professor of political science at Furman University in Greenville, S.C. Evangelical Protestants recruit other evangelicals to run for office, as do theologically liberal Catholics, conservative mainline Protestants, and so forth.

The most visible examples of such alignments have occurred among mainline Protestants and Roman Catholics, according to D'Antonio and Tuch.

Most mainline Protestant denominations have taken a formal position supporting a woman's right to have an abortion, while the Catholic Church has steadfastly opposed any form of abortion. Yet some lawmakers affiliated with those faith groups in recent sessions have voted the other way.

From 1979 to 2003, mainline Protestant Democrats -- Episcopalian, Lutheran, Presbyterian, Methodist and United Church of Christ -- generally followed their church's teaching, increasing their abortion-rights votes by 13 percentage points, from 62 percent to 75 percent, according to the study.

During the same time, mainline Protestant Republicans in the Senate shifted from being split on abortion -- 45 percent for abortion rights and 55 percent against -- to being 80 percent antiabortion in 1996. Mainline Protestant Republicans in the House have remained steady -- 80 percent are against abortion rights, D'Antonio said.

Not surprisingly, Catholic Republicans remained overwhelmingly antiabortion during the period of the study, voting almost unanimously with the antiabortion position taken by the Vatican and by the U.S. Conference of Catholic Bishops, D'Antonio said.

At the same time, Catholic Democrats have evolved from being strongly in favor of abortion rights to overwhelmingly so.

On other key issues taken by the bishops, the roles are reversed, with Catholic Republicans opposing the positions Catholic bishops have taken and Catholic Democrats supporting them. Such issues include taxes, minimum wage, health care, removing sanctions against Cuba and nuclear weapons.

In interviews and in their study, which has not been published, D'Antonio and Tuch said they are concerned that the polarization of Congress will undermine the country's two-party system, which was designed for consensus building.

As ideology has taken over and policy debates have become uncompromisingly brutal, many moderate Americans give up on the political process as potential officeholders or voters, D'Antonio said. "A significant portion of the public has been turned off by the political system, leaving the extremes to fight it out."

Sociologists Steven Tuch, left, and William D'Antonio completed an extensive survey of congressional voting patterns over a 23-year period.

Professors Tuch, left, and D'Antonio found in their study that on controversial issues, there is currently little middle ground among members of Congress, unlike among the general population.