At MOJ, Michael Moreland notes:

Upon the passing of Senator Daniel Inouye of Hawaii (a war hero from WWII, leaving Senator Frank Lautenberg of New Jersey the sole remaining WWII veteran in Congress), Senator Pat Leahy of Vermont becomes the President pro tempore of the Senate and, I believe, the first Catholic to hold the position. Indeed, if Senator John Kerry is nominated and confirmed to be Secretary of State, the four offices in line of succession to the presidency will all be held by Catholics: Vice President Joe Biden, House Speaker John Boehner, Senator Leahy, and Secretary Kerry.

UPDATE: My original post had a "slight correction" to Moreland's post, noting that Sen. Allan J. Ellender (d. 1972) of Louisiana was actually the first Catholic president pro tempore of the Senate.But Michael has done real research, and adds:

Ellender was baptized a Catholic but later listed himself as a Presbyterian and belonged to no church in his adult life. See Thomas Becnel, Senator Allen Ellender of Louisiana: A Biography (LSU Press, 1995), 149-50. But I take the modification to my earlier post.

Still, with six Catholics on the Supreme Court, Charles C. Marshall must be rolling in his grave. And the poor U.S. bishops -- they say Catholics haven't had it this bad since the persecutions of Diocletian. That could be a tougher sell now than it was in November.