Keable, who left PPS at the start of this year after five years as MD, described TSCG as an exponent of 'indirect lobbying'.
Rather than targeting politicians and policy makers direct, it aims to use 'people power' to change politicians' minds on controversial land-use battles.
Keable cited the Gambling Bill's dilution to allow for only one 'super-casino' as an example where people power was mobilised successfully in the UK through interest groups and media.
'Government, legislation, the media spotlight and Nimby pressure are making this a fast-growing sector,' he said.
Launched in 1983, TSCG has offices in Boston, Washington DC, Chicago, Nashville and San Francisco, with £9m of fee income. Keable heads a three-strong London office, reporting to TSCG chairman and CEO Mike Saint.
Regarding Keable's exit from PPS, the agency issued the following statement: 'An article appeared in PRWeek (25 March) about the restructuring of PPS and the departure of Keable. PPS wishes to emphasise that his performance as MD has never been in question. He served the company well during significant change and many challenges. We wish him well in future ventures.'