Comment by andrewla

Yes, you're right, they could just do this, and then spend the rest of their term thrown out of Congress and rotting in prison.

That is, they can hold such a lunch, but they can't pocket the money personally or even have the money contributed towards their campaign. Pocketing the money personally is just bribery, having it contribute to their campaign would violate the contribution limits (typically ~$10,000 per election max, though less than that in reality because the limit is split between primary and general).

You're right to be slightly suspicious because these lunches do exist. But they don't go to the campaign or the candidate, they can go to PACs and SuperPACs or to the party. In tern, PACs and SuperPACs cannot give this money to the candidate or the campaign (same limits as above with higher reporting requirements) and cannot coordinate campaign messaging.

This is not to say that corruption doesn't exist -- just that this simplistic form of it does not exist. You'll have to dig deeper than looking at lobbying activities to find the actual corruption, and it's usually masked in revolving door arrangements, speaking fees, and bulk sales of vanity books. But none of that can go through the typical lobbying system because it's too tightly regulated and tracked.