It's true there's only been one Catholic president. On the other hand John Kerry's religion was scarcely mentioned during the last election, and the proportion of Catholics in the U. S. Congress is roughly the same as their proportion in the population as a whole. There were 150 Catholic members in the 107th Congress, making 28%.

They do skew toward the Northeast, the Great Lakes, and California.

It was the (barely) Protestant Ronald Reagan who named the first U. S. ambassador to the Holy See; previously relations had been handled by an ambassador without portfolio. Possibly the Protestant "domination" of the U.S. results from nearly two-thirds of the people adhering to one or another Protestant denomination. But then you're lumping Quakers and Methodists with Evangelical Lutherans and Southern Baptists.

I disagree that the Russian and Greek Orthodox churches are the 'main competitors' of Catholicism. I suspect more people have been born Muslims in the 26 years of the Pope's reign than are Russian Orthodox.    And while the Russian Orthodox church is the nominal faith of 90% of Russians, in fact the active membership is much lower, as with alleged Catholics in France. And in Latin America, the Catholic church is confronting conversion to various forms of evangelical Protestantism.