Once a legislator has been elected, he/she is presented with a wide latitude for for public service along with the potential for abuse and corruption. Today, almost every senator and congressperson is on the take from Big Interests and the monied, privileged and the powerful. This includes every Republican and all but a small handful of Democrats who have sworn off corporate PAC money.
The means that all congressional Republicans and, to a lesser extent, Democrats, are responding more to the people who are bribing them than those who elected them. Once elected, a lawmaker need no longer worry about keeping promises to the voters but if he/she stops responding to donors and lobbyists, that money dries up. Standard practice is to publicly claim adherence to campaign promises to an electorate which keeps itself largely ignorant of how the official actually voted.
Getting rich is now the only goal of too many who run for office. Their official activities are for sale. And the result has been the replacement of Democracy with Plutocracy.