What should determine a congressman's vote? The will of the people?

Congressmen serve at least half a dozen masters: (1) the individuals who bankroll their campaigns; (2) the party they belong to, which can determine whether they get primaried or not, and which may demand that they sacrifice the interests of their constituents in some instances for overall Party demands; (3) the welfare of the voters in their district, (5) the wishes of the voters in their district, and (6) the congressman's personal principles. 
 
And what determines the wishes of voters in a congressman's district? What if they've been the target of a clever, lavishly funded propaganda campaign by special interests who exploit those voters' fears and prejudices? 
 
In Nazi Germany, I'm sure a majority of voters believed everything bad was all the fault of the Jews. Here, today, for example, a majority of Republican voters still believe that Saddam Hussein was behind 9/11. They didn't just think that up. They were victimize by Republican demagoguery. 
 
And today a majority of American voters don't accept the fact that we're in the midst of dangerous global climate changes caused by human activity. They don't accept this because Exxon and the Koch brothers and other major petrochemical firms have spent a huge amount of money (from a non-billionaire viewpoint) in getting people to believe that. 
 
It isn't in their best interests to believe that. And their Congressman has a moral obligation to help convince them otherwise and to vote himself accordingly.  
 
With guns, here again a small group of immensely wealthy men involved in gun manufacturing and sales have spent a whole lotta money propagandizing people about guns, leading them to believe all sorts of patent falsehoods about guns and gun violence.  
 
A Congressman will feel immense pressure to cave in to the wishes of his patrons and his propagandized, badly misled voters. To think it's moral for him to cave in to this is, well, anything but moral.