Pretty much every politician in the US proclaim their Christianity. They do this because a supermajority of the US population professes at least nominal Christianity. The mere statement by a politician that they are Christian carries precious little information, because if they were not, they generally would say they were anyway. My gut tells me that an awful lot of politicians in the US are functionally atheists and this isn't limited to the Democratic party or 'Catholics in Name Only'. They simply profess a faith because it is death electorally if they do otherwise.
About the only politicians you can probably take at face value when they declare a religious affiliation are the ones of non-mainstream faiths in the US---like say a Buddhist or a Mormon. Much talk is advanced on whether Obama, for instance, is a secret Moslem. I don't think he is, but I don't think he's a Christian either. Obama worships Obama, and seems to want everyone else to also. Christians and others who care about religious values in the political arena in the US should pay much more attention to a politician's track record of voting on the issues that matter to them and put much less stock in their nominal affiliation.