History suggests that only one thing is certain about Tuesday's crowded Democratic primary election for district attorney in Philadelphia: The vast majority of eligible voters will ignore it.

A review of voter turnout for the last quarter-century shows that the bulk of Democratic voters don't bother to cast a ballot when given the chance to chose the city's top prosecutor, even when there is a contested primary election and lots of money is being thrown around to try to capture their attention. This, in a Democratic town, means a sliver of the electorate is making the final selection.

Seven Democrats are vying to lead the District Attorney's Office, mounting robust and in some cases expensive campaigns. But that won't necessarily translate into turnout.

"You wouldn't lose any money by betting turnout is somewhere in the low teens on this one," David Thornburgh, CEO of the good government group  Committee of Seventy, said Tuesday. "But there's enough interesting moving pieces that might suggest it'll go higher than conventional wisdom would expect."

Thornburgh hopes that some post-presidential election frustration among Democrats prompts a higher turnout and that an open field with no incumbent could inspire some excitement.

"If you look back at the last few years, I think in general the criminal justice system and various controversies swirling around it have elevated the importance of that office," he said. "So as compared to other elections, I think the DA's race, maybe, has come into focus in a way that it hasn't recently."

If Tuesday's primary has light turnout, it won't be for lack of trying -- and spending -- by independent political action committees pumping cash into efforts to interest Democrats.

Philadelphia Justice and Public Safety, a PAC supporting civil rights lawyer Larry Krasner with $1.45 million from billionaire George Soros, on Monday reported spending $302,476 to air television commercials, mail campaign literature, and pay people to canvass neighborhoods to talk to voters.

Building a Better Pennsylvania Fund, a PAC supporting former Assistant District Attorney Jack O'Neill and funded by local building trades unions, on Tuesday reported spending $85,840 for cable television ads to oppose Krasner's campaign. That was on top of $144,340 the PAC reported spending on Friday for pro-O'Neill television commercials.

Historically, elections for district attorney have attracted among the lowest turnouts of any cycle.

Lynne M. Abraham became district attorney in 1991 when a group of her fellow Common Pleas Court judges selected her to replace Ronald D. Castille, who resigned   to run for mayor.

Abraham was unopposed in the primaries when she sought reelection in 1993 and 1997. Turnout in the primary was 13 percent in 1993 and 14 percent in 1997.

Historical primary turnout figures include Democrats and Republicans. Democrats in Philadelphia hold a 7-1 voter registration advantage over Republicans.

The 2001 primary was a different story for Abraham.  Some African American community leaders, upset that she had opposed a local judge's 1998 nomination to a federal job, recruited former City Commissioner Alex Talmadge to challenge her.

That set off a bruising battle, rife with racial politics, that drew more attention than a typical race for district attorney.  Voter turnout in the primary was still just 22 percent.

Abraham faced another challenge in 2005 from Seth Williams, who had once served under her in the District Attorney's Office.

Just 14 percent of eligible voters cast a ballot in that primary.

A similar open multi-candidate field did not seem to alter the situation in 2009. When Abraham decided not to seek reelection that year, five Democrats sought to replace her. Voter turnout in the primary was 12 percent.

Williams won the primary with just 43,672 votes  -- about 41 percent of the vote. A majority is not necessary to win a primary, and no candidate is likely to attract a majority this time around.

"When you have this many competitors in the mix and they're all pulling from different groups, the winner is very likely to win with a plurality," City Commissioner Al Schmidt said. "The winner could win with 35 percent."

In that example, even if turnout was 15 percent, roughly 5 percent of registered Democrats would have voted for the winner.

Thornburgh lamented such an outcome.

"I think there's a basic principle that says to give that person really the full confidence of the people and vice versa, you should have to win by a majority," Thornburgh said.

With the Democrats' voter registration advantage, it is virtually guaranteed that the primary winner will win the general election.

That was true for Williams in 2009, when he won the general election with 92,273 votes -- just 8.7 percent of all the voters registered in the city during that election, in which turnout was just 12 percent.

Williams coasted to a second term in 2013, running unopposed in a primary that drew 9 percent turnout and defeating a Republican in a general election that drew 12 percent turnout.

He dropped his bid for a third term in February and was indicted in March on 23 federal charges, accused of taking $34,145 in bribes from two businessmen and stealing $20,319 meant for the care of his elderly mother.

Williams, who has pleaded not guilty and denies the charges, is scheduled to go on trial May 31, just two weeks after the primary.

In addition to Krasner and O'Neil, candidates for district attorney are former first assistant district attorney Tariq El-Shabazz; former city and state prosecutor Michael Untermeyer; former Municipal Court Judge Teresa Carr Deni; former federal prosecutor Joe Khan; former Managing Director Rich Negrin; and the lone Republican, Beth Grossman, who worked in the District Attorney's Office for 21 years.