Complex sentence: Part of the sums paid by the tribes for lobbying were paid to the American International Center, an organization presenting itself as a think tank headed by David Grosh, a lifeguard on the Delaware shore who operated it from his beach house.
Split into simpler sentences: Part of the sums paid by the tribes for lobbying were paid to the American International Center, a purported think tank headed by David Grosh. In fact, he was a lifeguard on the Delaware shore who "operated" the organization from his beach house.