The Founding Fathers took the issue of accountability very seriously. Ultimately, elected officials are responsible to those they were elected to represent.
Members of the U.S. House of Representatives, for example, are elected to serve two-year terms. U.S. Senators, in contrast, are elected to six-year terms. This was an important distinction in that the authors of the Constitution recognized that members of the House would be more accountable to their constituents by virtue of the frequency by which they are subject to elections. This has the intended consequence of ensuring that these representatives remain closely tied to their constituents.
Senators, as stated, are elected to six-year terms. This provides them greater latitude in drawing distinctions between parochial considerations and the greater national interest. By not being subject to reelection every two years, senators are more buffeted from the short-term consequences of votes that may not benefit their constituents at the expense of other districts or states, or the nation as a whole.
The Founders recognized that an informed citizenry is crucial to the proper functioning of a democracy (and of a representative form of government, or a republic).
Ultimately, members of Congress are held accountable by the people of their districts in elections. The people decide whether the actions of the members warrant sending them back to Congress for another term. This is the clearest way of holding someone accountable for their votes.
However, there is more to it than that. How, for example, do people in the districts find out about the members’ actions so that they can decide how to vote? In general, this happens through the media, from friends (sometimes through social media) and through interest groups. These are vital parts of the mechanism for holding members responsible. If it were not for these mechanisms, the people would not know enough to hold the members of Congress responsible.
So, we can say that members of Congress are held accountable through the electoral system. However, we must also note that the mechanisms that allow people to know what their representatives have done are also important in holding the members responsible.