We measure the branching fraction of the top quark to longitudinally and right-handed polarized W bosons, F0 and F+, using approximately 200  pb−1 of pp collisions collected by the CDF II detector. We analyze two quantities sensitive to the W helicity: the invariant mass of the charged lepton and the bottom-quark jet in the decay t→Wb→ℓνb (where ℓ=e or μ), and the transverse momentum of the charged lepton. Constrained fits yield F0=0.74+0.22−0.34, and F+<0.27 at the 95% confidence level. These measurements are consistent with the standard model predictions.