The 3.5 billion doesn't go to congress people. It goes into reelection coffers. Or as the sibling points out into electing-others-coffers.
We like to believe in free-and-fair elections, but in most cases the options are "unknown names". Most people vote party, not person.
The real "electing" happens at the primary level even all candidates are the same party. At this level money talks (through advertising.) Basically its a most-money-wins setup.
Congress is the pinicle of the iceberg you can see, but the real action happens at state level, in primaries, etc. That's why you end up with "2 bad choices" come election time.
The only way to remove money from politics is to ban TV and internet political advertising. (And it won't shock you to discover TV and internet think that's a bad idea.) Frankly, I'm not even sure that would help.
Politics bows to moneyed interests because politics is built on access to advertising money. RFK didn't drop out because he lost interest, he dropped out cause he ran out of money. He then sold his endorsement to the highest bidder.
Ultimately US society values money above all else. So it's not surprising that money is at the root of US politics. As far as the system is concerned this is not a bug, it's the killer feature.
An improvement would be if the number was lower because hired lobbyists were less effective. We should undo Citizens United with legislation so that concentrated wealth can't be used to bully politicians as easily, giving them the freedom to act on voters needs. If we treat unlimited political spending as the corruptive force it is instead of a fundamental right, things wouldn't be perfect but they would surely be better. Sadly, a corrupt legislature is unlikely to legislate away corruption. Campaigning on reversal of the CU ruling brought massive popular support and individual donations to Bernie Sanders' presidential campaign, but IMO was a also a big (maybe biggest) reason he was treated as untenable by the establishment.