A president who sees the possibilities of the moment can rack up achievements that seemed foreclosed. According to Robert Caro's account in "The Path to Power," Johnson knew instinctively after John F. Kennedy's assassination that he could use the slain president's memory to pile up successes in Congress. Caro quotes Johnson discussing the mechanics of his strategy: "I had to take the dead man's program and turn it into a martyr's cause." When Johnson addressed Congress days after Kennedy's death, he did just that: "[No] eulogy could more eloquently honor President Kennedy's memory than the earliest possible passage of the civil rights bill for which he fought so long."
Voters need to appreciate these currents almost as much as presidents in order to accurately assess a president's political performance or a challenger's promises. How steep was the opposition that a president faced? How boxed in was his agenda by the unexpected emergencies of the day? Did these fire alarms increase his political capital or drain it? Is the challenger offering pie-in-the-sky promises? Will his proposals face public fatigue, or are people hungry for sweeping change?
Looking at a presidency this way has one other advantage: Moments of greatness can come into full view. You can identify those instances when a president faced great obstacles and plowed ahead despite the high political price he would pay. That's the only way to describe Johnson's decision on the Civil Rights Act of 1964, even though he knew it would permanently cost his party support in the South. When George H.W. Bush supported a budget deal in 1990 that broke with his "no new taxes" pledge, it may have cost him his re-election.