  We develop a method of transverse momentum dependent (TMD) operator expansion
that yields the TMD factorization theorem on the operator level. The TMD
operators are systematically ordered with respect to TMD-twist, which allows a
certain separation of kinematic and genuine power corrections. The process
dependence enters via the boundary conditions for the background fields. As a
proof of principle, we derive the TMD factorization up to the next-to-leading
power $(\sim q_T/Q)$ at the next-to-leading order for any process with two
detected states.
