Instead of running for reelection in 1956, Eisenhower publicly announces he will not run for reelection well in advance of the election. Say the POD is worsening health, and so he decides to get out.
So who are the nominees for the parties? Without the incumbent Ike, the Democrats have a opportunity and so might not renominate Stevenson. While the Republicans would likely pick Nixon as the VP, thought Eisenhower did have preferences for others. And perhaps the conservative wing could try to seize the reins.
And who would win the election? And their impact compared to a second Eisenhower term.