There are a range of factors that affect how Congressmen vote; most notably in the USA this is their state, their personal beliefs and their party, most probably in that order.

Looking at the religious composition of the 114th (and 113th) Congress’, it is easy to see why Obama had such a battle over abortion in the Affordable Care Act. This article goes on further and looks at religion by state in the USA. For those students looking for evidence of factors that influence voting other than the party, or indeed for examples of how important the state is in terms of voting, this provides some excellent data.