No they don't. They look at how their party votes, how their donors want them to vote, what is expected of them, and then if there is time then they may have some aids fill them in on what a bill may-or-may-not do. These "positives" and "negatives" are really not even considered, and they can be subject to individual interpretation. Such as a bill to legalize same-sex marriage. Depending on who you are, for most people, that in itself is either a positive or negative thing.
And, BTW, America is not a democracy.
That is not "we." We were mostly and largely against the wide and generalized language of the Patriot Act, especially the part that allowed for sweeping surveillance without a warrant, but it happened anyways. We do not like such invasions of privacy, but the politicians make it happen. We did not get to decide on hooking cell phones up to a machine or not, yet the politicians are. We are against the outlandish money in politics, but it happens anyways. We tend to believe groups like ALEC are criminal, but yet many politicians are firmly aligned with the group. We have for sometime now have strongly disapproved of Congress, but yet nothing has changed.