Is the top vote-getter in a Texas primary election most likely to win their runoff? Usually, but not always.

Today’s Texplainer question was inspired by reader Ralph Gauer.

Hey, Texplainer: In state runoff elections, which candidate more frequently wins? The top vote-getter in the primary or the runner-up?

As last month’s primary election results rolled in, Democratic congressional hopeful Christine Eady Mann followed three TVs and several computers and phones inside a Georgetown restaurant with family and dozens of supporters.

The family physician didn’t win. She didn’t even get the most votes. But she garnered enough support to face Air Force fighter MJ Hegar in next month’s runoff election to decide who will challenge U.S. Rep. John Carter, R-Round Rock, in Texas’ 31st Congressional District. And while the historical odds aren’t in her favor to win, Mann is confident she’ll be among the minority of second-place primary finishers who win their runoffs.

“We’re targeting certain voters and certain demographics, and we’re just being very diligent in reaching out to these people,” said Mann, who won 33.5 percent of the primary vote compared to Hegar’s 44.9 percent.

In runoffs since 2002, candidates like Mann have a 39 percent chance of coming from behind and besting their competitors, according to data collected by Jeff Blaylock, the publisher of Texas Election Source.

Meanwhile, the fact that first-place primary candidates typically win their runoffs is good news for Gina Ortiz Jones. The former Air Force intelligence officer finished 24 percentage points ahead of Rick Treviño in the Democratic primary for Texas’ 23rd Congressional District, an area which stretches from San Antonio to El Paso.

“We’ll continue doing what we’ve been doing and not assuming anything away,” Jones said. “We’re very much working as hard as we have been.”

Blaylock tracked runoff races where there was no incumbent back to 2002. While the top vote-getter before the runoff has a 60 percent chance of clinching the nomination, those odds change depending on what their lead was.

Looking at statewide, legislative, federal and State Board of Education seats, Blaylock found the following outcomes for candidates who finished first in their primaries:

• Those who won more than 40 percent of the vote and beat their closest opponent by more than 10 percentage points won their runoff 82 percent of the time. If the lead was under 10 percentage points, that chance of victory drops to 55 percent.

• Candidates who got first in their primary with less than 40 percent of the vote and a lead of more than 10 percentage points won their runoff 87 percent of the time; those with less than a 10 percent lead won the runoff 45 percent of the time.

• A lead of 15 percentage points or more in the primary translates to a 90 percent chance of winning the runoff.

But there’s some good news for some second-place finishers: If they emerge from the primary within 2.5 percentage points of the leader, those runners-up win 62 percent of the time in the runoff.

“The lead is the most important factor,” Blaylock said. “If you were more than 10 points ahead, historically, you’re probably going to stay there. If the race is closer than that, there’s a lot more opportunity for things to shift.”

Of course there are some notable exceptions. After placing second in their primaries, both Republican U.S. Sen. Ted Cruz and former U.S. Rep. Blake Farenthold of Corpus Christi won their respective runoff races against David Dewhurst in 2012 and James Duerr in 2010.

When there’s an incumbent involved, it’s a slightly different story.

“If an incumbent is in a runoff, it’s usually for a reason,” Blaylock said. “And it’s usually not a good reason.”

He tracked Texas House and Senate races going back to 1996 and found that just five of the last 25 legislative incumbents forced into a runoff have won.

“If an incumbent can’t reach 50 percent, then a majority out there wants them to be replaced,” said Mark Jones, a Rice University political scientist.

No legislative incumbent has won a runoff after finishing second in the primary since state Sen. John Whitmire, D-Houston, in 1992, Blaylock said.

Mark Jones said second-place primary finishers tend to be at a disadvantage during runoff season since most donors and organizations want to support the person they think will win.

But there are ways for them to come out on top.

“The one sort of exception is if the second-place candidate can make one of two credible arguments: The first one is that the first-place candidate is an ideological outlier — either they’re far too conservative or far too liberal for the district,” Mark Jones said.

The other argument second-place finishers can make, he said, is that they lost because the vote in their district was split among several candidates.

If history is any guide, May primary runoff turnout will be low: Just 16 percent of the eligible voting population cast a ballot for the March 6 primaries, and typically half of the voters who cast a ballot in the primaries will show up to a runoff.

The biggest key to winning a runoff may not be name ID or money, experts said. It’s getting the people that voted for you the first time to come back to the polls.

“Almost every runoff candidate is going to lose voters from the primary election,” Blaylock said. “So it’s less about trying to get other supporters to come out and vote for you than trying to get the people who voted for you the first time to come back.”

The bottom line: If you earned first place in your primary race, the odds are in your favor for next month’s election. But if you’re an incumbent who was forced into a runoff election, historically, it’s harder to pull off a win.

Disclosure: Rice University and Jeff Blaylock have been financial supporters of The Texas Tribune, a nonprofit, nonpartisan news organization that is funded in part by donations from members, foundations and corporate sponsors. Financial supporters play no role in the Tribune’s journalism. Find a complete list of them here.