In years when both parties had statewide primary runoffs, turnout in the second round of voting averaged almost half of turnout in the first round. On average, the runoff got a vote for every two in the primary.
Republicans tend to be worse about coming back. In elections with a statewide runoff, their average runoff turnout was 27.3 percent of their average primary turnout. For Democrats, the corresponding number was 34.9 percent.
Caveats abound: Every election is different and has funny bounces; it's unusual to have equal reasons for Democrats and Republicans to return to the ballot box in the same year; and not all statewide elections are equal, with a U.S. Senate race one year and a Court of Criminal Appeals race the next to drive out the runoff vote.
The high mark for Republicans over the last 20 years was in 1998, when the runoff drew 39.1 percent of the first round voters (not the same voters, necessarily, but that number). The GOP's low-water mark was in 2000, when one in five voters came back. For the Democrats, 2008 was the stinker, at 6.5 percent, and 1994 was the high mark, at 72 percent.
The mother of runoffs? Easy: It was 1993, when a May 1 special election for U.S. Senate boiled over into a June 2 runoff between Kay Bailey Hutchison and Bob Krueger. In that one, the dropoff from the first round of voting to the second was relatively small: the runoff drew 86.2 percent of the first round voters.
A little history
• In 1992, neither party had a statewide runoff race.
• The 1996 GOP runoff had Court of Criminal Appeals seats, but no top-of-ballot race to attract voter attention. The Democrats, that same year, were watching Victor Morales beat a couple of incumbent congressmen in the primary for U.S. Senate. That proved to be a bigger draw in the runoff.
• In 1998, the draw in the GOP runoff was an attorney general race between John Cornyn and Barry Williamson. Democrats didn't have a statewide runoff.
• In 2000, Republicans had a weak runoff showing statewide, with a court of criminal appeals race at the top of the ballot. Charles Gandy and Gene Kelly led the Republican ticket, vying for U.S. Senate; Kelly went on to face Hutchison in the general.
• Texas Democrats ran their well-financed "dream team" ballot in 2002, and the Senate runoff between Morales and Ron Kirk drove turnout that year. Republicans had a state Supreme Court race at the top, and lower turnout in the runoff.
• The Republicans had a Railroad Commission runoff between Victor Carrillo and Robert Butler in 2004 to top their ballot. Democrats didn't have a statewide race in their runoff.
• The 2006 runoff on the Republican side was, once again, a low turnout court race. On the Democrats side, it was a U.S. Senate race, but one in which neither candidate has the financial ability to run a strong statewide campaign to raise voter interest.
• The 2008 primaries were a big draw, with the Democratic candidates for president tangled in an undecided race and Republican voters energized by the amount of political activity, even though much of that was in the other primary. The Republicans didn't have a statewide runoff that year. On the Democratic side, there was a statewide runoff, but after the Hillary Clinton/Barack Obama match, it was a Peewee League pairing for Railroad Commission between Dale Henry and Mark Thompson.
• On a smaller scale, that's what happened with the GOP in 2010; a three-way gubernatorial primary with Hutchison, Rick Perry, and Debra Medina drew voters in March, but the top race in the runoff was a Supreme Court matchup. It wasn't much of a draw. On the Democratic side, no statewide race went to a runoff.
This year's notes
• All told, there are 37 runoffs on July 31, including 25 in Republican primary and a dozen in the Democratic primary.
• There are five statewide runoffs, four of them in the Republican primary and one in the Democratic primary.
• There are 11 congressional runoffs, five Republican, six Democratic.
• Three SBOE runoffs, two Republican, one Democratic.
• One Republican Senate runoff.
• And there are 17 runoffs in the Texas House, 13 Republican and four Democratic.
A mildly daring speculation
• 1.4 million Texans voted in the Republican primary this year. If the party hits its average runoff return, that would translate to 384,114 voters on July 31.
• Just over 590,000 Democrats voted in May; if they get their average runoff turnout, they'll have 205,973 voters this month.
• If both parties match their historic highs, nearly a million Texans will vote this month. If it's a time for historic lows, the total number would be 318,475.