The story from The Pew Forum:

A few tidbits

Collectively, Protestants account for more than half (54.7%) of the 111th Congress, about the same proportion as their share of the U.S. adult population (51.3%). But American Protestantism is very diverse and encompasses more than a dozen major denominational families, such as Baptists, Methodists and Presbyterians, all with unique beliefs, practices and histories. When these Protestant denominational families are considered as separate religious groups, Catholics are the single largest religious group in the 111th Congress. Catholics, who account for nearly one-quarter of the U.S. adult population, make up about 30% of Congress. Indeed, the number of Catholics in Congress is two-and-a-half times the size of the next largest religious group, Baptists, who make up about 12% of the members.
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Differences become much more pronounced at the party level. While 70.8% of congressional Republicans are Protestant, fewer than half of Democrats (43.6%) belong to Protestant denominational families. On the other hand, the share of Democratic members who are Catholic (36.6%) is much greater than the number of Catholic GOP members (21%). And while Jews make up 13.4% of all congressional Democrats (including two independents who tend to caucus with the Democrats), they account for just 0.9% of congressional Republicans, with one Jewish Republican in the House and one in the Senate.
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In many ways, the changes in the religious makeup of Congress during the last half-century mirror broader changes in American society. Congress, like the nation as a whole, has become much less Protestant and more religiously diverse. Indeed, the total percentage of Protestants in Congress has dropped from 74.1% in 1961 to 54.7% today, which roughly tracks with broader religious demographic trends during this period. As recently as the early 1980s, nearly two-thirds of Americans identified themselves as Protestants. In the recent Landscape Survey, the number of self-identified Protestants dropped to 51.3%.

Not surprisingly, many of the major Protestant denominational families have lost ground in Congress in the past 50 years. Methodists, who made up nearly one-in-five members (18.2%) of the 87th Congress, which was seated in 1961, make up only 10.7% of the 111th Congress. The share of Presbyterians and Episcopalians also has dropped significantly during this period, from 13.7% and 12.4%, respectively, to 8.1% and 7.1% in the new Congress. Finally, Congregationalists have dropped from 5.1% to 1.1% during this period.

A few Protestant groups have fared somewhat better, however. From 1961 to today, the number of Baptists in Congress has remained roughly the same, at around 12%, as has the Lutheran share (around 4%).