Vox - Members of Congress now make $174,000 a year ... but usually they can at least quintuple that salary by switching over to lobbying once they retire. And many of them do just that.
A new study by three political scientists has some good data on the trends. Jeffrey Lazarus, Amy McKay, and Lindsey Herbel went all the way back to 1976 to see who went on to lobby after leaving Congress. Their results are reported in a new article, "Who walks through the revolving door? Examining the lobbying activity of former members of Congress," in the journal Interest Groups & Advocacy. The below image comes from that article.
Back in the 1970s and '80s, it was relatively rare for former Congress members to become lobbyists. This makes sense, since the lobbying industry was not nearly as big then. The real growth of lucrative Washington lobbying has been since the 1990s.... Now reported lobbying is a $3.2 billion-a-year activity.
The obvious caveat in these measures is that they only represent registered lobbyists. Many former members prefer not to register, but still do work that looks very much like lobbying, with former Sen. Tom Daschle (D-SD) being a prominent example. Political scientist Tim LaPira estimates that there's probably twice as much lobbying in Washington as shows up in disclosures.
And prior to 1995, when Congress passed the Lobbying Disclosure Act, registration requirements were even looser, though registration rules did exist.