Nixon’s deal required Nixon to back it up - as in the response to the NVA’s “Easter Offensive”. When the Left took out Nixon, they had effectively won the war for the NVA.
Right - Ford wanted to honor the agreement, but congressional Dems voted down the funds/arms. Ford was furious; he knew we had betrayed South Vietnam. In fact, the terms Nixon negotiated made the fall of Saigon inevitable; the South Vietnamese president was furious with him (allowing the NVA to remain in parts of South Vietnam while we left, for instance).