Winning in the Runoffs: How First-Place Primary Finishers Fare

Last week, we looked at the historically poor success rate of incumbent legislators who get forced into runoffs. Since 1996, just about one out of every four has prevailed. The historical picture is especially bleak for those incumbents who came in second place. None have prevailed in three decades.But what about open-seat runoffs? How often does the candidate who finished in first in the primary (P1) prevail over the candidate who came in second (P2)?Overall, 60% of P1s prevailed – that’s 3 out of every 5 – since 2002, which of course means P2s prevailed 40% of the time. Broken out by party, 57% of Republican P1s and 64% of Democratic P1s have gone on to win their runoffs.But that success rate varies quite a bit depending on two factors: P1’s raw percent of the vote in the primary and the percentage point margin between P1 and P2. There are significant differences in success rates for P1s who finished with more than 40% of the primary vote versus those who finished with less than 40%; and between P1s whose primary margin was in the double digits versus those who finished less than 10 percentage points ahead of P2.Looking just at vote percentage, P1s who received 40% or more of the primary vote have prevailed 69% of the time, while P1s who received less than 40% have won their runoffs 52% of the time. The historical success rate for P1s north of 40% is unaffected by party. Republican P1s who received 40% or more in the primary went on to win 68% of the time versus 70% of the time for Democrats.

The initial margin between P1 and P2 has historically had a bigger impact. Regardless of the vote percentages either candidate received, P1s who finished 10 or more percentage points ahead of P2 have won 81% of the time, which is 4 out of every 5. That winning percentage is essentially the same among Republicans (83%) and Democrats (78%).Closer primaries tend to produce more volatile runoff results. Since 2002, it’s been essentially a coin flip when the two candidates were separated by single-digit margins in the primary. Regardless of the candidates’ vote percentages in the primary, P1s with a margin of fewer than 10 percentage points have prevailed 47% of the time. The spread between the parties is a little larger here: 42% of Republicans and 54% of Democrats.So, the answer to the question, “How much of an advantage does a first-place finisher have in the runoffs for open seats?” is, “It depends.” Did the P1 receive at least 40% of the vote in the primary? Did the P1 finish at least 10 points ahead of their runoff rival in the primary? If the answer is “yes” to both, then it’s a big advantage – 83% success rate. If the answer is “no” to both, then it’s less than a coin flip – 46% success rate.And if It’s “no” to both, and it’s a Republican runoff, then the advantage goes to the P2, who went on to upset the P1 62% of the time.