We use the three lepton and missing energy trilepton signature to search for chargino-neutralino production with 2.0  fb−1 of integrated luminosity collected by the CDF II experiment at the Tevatron pp¯ collider. We expect an excess of approximately 11 supersymmetric events for a choice of parameters of the mSUGRA model, but our observation of 7 events is consistent with the standard model expectation of 6.4 events. We constrain the mSUGRA model of supersymmetry and rule out chargino masses up to 145  GeV/c2 for a specific choice of parameters.