WASHINGTON — The share of U.S. adults who describe themselves as Christians has been declining for decades, but today’s Congress is about as Christian as it was in the early 1960s, according to a new analysis by the Pew Research Center.

The survey finds that among members of the new, 115th Congress, which took office Tuesday, 91 percent describe themselves as Christian. That compares with 71 percent of U.S. adults who consider themselves Christian.

The 91 percent figure is nearly the same percentage as the 87th Congress, which served from 1961 to 1962 and is the earliest year for which comparable data are available. That Congress was 95 percent Christian.

Among the 293 Republicans elected to serve in the new Congress, all but two identify as Christians; there are two Jewish Republicans in the House of Representatives — Lee Zeldin of New York and David Kustoff of Tennessee.

Democrats in Congress also are overwhelmingly Christian, at 80 percent. But the 242 Democrats in Congress also include 28 Jews, three Buddhists, three Hindus, two Muslims and one Unitarian Universalist — as well as the only member of Congress to describe herself as religiously unaffiliated, Rep. Kyrsten Sinema of Arizona.

In addition, all 10 members of Congress who decline to state their religious affiliation are Democrats.

Like the nation as a whole, Congress has become less Protestant over time. The percentage of Protestants in Congress dropped from 75 percent in 1961 to 56 percent today. During this period, the share of Roman Catholics in Congress went from 19 percent to 31 percent.

The analysis finds that some religious groups, including Protestants, Catholics and Jews, have greater representation in Congress than in the general population.

Jews, for example, make up 2 percent of the U.S. adult population but account for 6 percent of Congress. Other groups — including Buddhists, Mormons, Muslims and Orthodox Christians — are represented in Congress in roughly equal proportion to their share in the U.S. public.

The group that is most notably underrepresented is the religiously unaffiliated. This group — also known as religious “nones” — now accounts for 23 percent of the general public but just 0.2 percent of Congress.