Yes, he would have been able to cobble together the old New Deal coalition for a few more years. The New Left really wouldn't have anywhere to go but to sit out 1972, but Humphrey would have held on to labor (for whom it can be expected he would have produced something) and thus a part of the socially conservative voting bloc that defected to Nixon. A big factor is what happened with Vietnam. If he produced a peace treaty and meaningful troop withdrawals without any real stench of defeat, that, plus what was a decent economy, should get him reelected.