  We consider a relativistic particle model in an enlarged relativistic phase
space M^{18} = (X_\mu, P_\mu, \eta_\alpha, \oeta_\dalpha, \sigma_\alpha,
\osigma_\dalpha, e, \phi), which is derived from the free two-twistor dynamics.
The spin sector variables (\eta_\alpha, \oeta_\dalpha, \sigma_\alpha,\
osigma_\dalpha) satisfy two second class constraints and account for the
relativistic spin structure, and the pair (e,\phi) describes the electric
charge sector. After introducing the Liouville one-form on M^{18}, derived by a
non-linear transformation of the canonical Liouville one-form on the
two-twistor space, we analyze the dynamics described by the first and second
class constraints. We use a composite orthogonal basis in four-momentum space
to obtain the scalars defining the invariant spin projections. The
first-quantized theory provides a consistent set of wave equations, determining
the mass, spin, invariant spin projection and electric charge of the
relativistic particle. The wavefunction provides a generating functional for
free, massive higher spin fields.
