In an excellent paper titled, Revolving Door Lobbyists, Jordi Blanes i Vidal, Mirko Draca and Christian Fons-Rosen use data on the lobbying revenues of ex-Senate staffers to show:

[L]obbyists connected to US Senators suffer an average 24% drop in generated

revenue when their previous employer leaves the Senate. The decrease in revenue is out of line with

pre-existing trends, it is discontinuous around the period in which the connected Senator exits Congress

and it persists in the long-term. The sharp decrease in revenue is also present when we study separately

a small subsample of unexpected and idiosyncratic Senator exits. Measured in terms of median revenues

per ex-staffer turned lobbyist, this estimate indicates that the exit of a Senator leads to approximately

a $177,000 per year fall in revenues for each affiliated lobbyist….We also find evidence that ex-staffers are more likely to leave the lobbying industry

after their connected Senator or Representative exits Congress.

Here is the key figure showing the drop in revenues at the time the Senator exits: