How many Nashvillians will vote in transit referendum? It's anyone's guess.

Predicting how many people will head to the polls can usually be done with a fair amount of accuracy.

But turnout in Nashville's hotly debated transit referendum election has become one big mystery.

After months of furious back-and-forth on the issue, early voting begins Wednesday on Nashville's transit plan — a controversial proposal to increase four taxes, including the sales tax, to pay for a $5.4 billion combination of light rail, rapid bus and bus system improvements.

The referendum is tacked on to a local primary ballot that typically has the city's lowest turnout of any election. The May 1 election features party primaries for sheriff, register of deeds, county clerk and other county races.

The most recent May primary election in 2014 generated a turnout of 38,804 voters. Just 23,136 people voted in the May 2010 election.

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But the dynamics are wildly different this year with the addition of the high-profile transit vote, which has been the subject of millions of dollars in campaign activity, including a deluge of television advertising in recent weeks.

Turnout is expected to be higher than those last two elections. But by how much?

"It's kind of hard to pick, but we're thinking somewhere around 100,000 people," said Davidson County Elections Administrator Jeff Roberts, referring to overall turnout. "But we really don't have a whole lot to base that on."

Turnouts of Nashville's past referendums

The election commission's 100,000 voter turnout projection — higher than projections of most other stakeholders — splits the difference between turnout of Nashville's two most recent major public referendums:

In May 1996, 125,913 Nashvillians voted in the "NFL Yes" referendum, which passed, to build a football stadium to lure the then-Houston Oilers to Nashville.

In January 2009, 73,912 people voted in the English-only referendum, with voters defeating a proposal to make English the official language of Metro government.

But unlike the upcoming transit referendum, the NFL stadium referendum was a stand-alone election with nothing else on the ballot. The English-only referendum had only one additional referendum on the ballot.

The selection in May for the transit referendum rather than higher-vote elections in August and November — when races for governor and U.S. Senate are on the ballot — was a strategy of transit proponents. The Metro Council approved the date and ballot language in February.

Transit proponents, opponents disagree on turnout projections

The Nashville for Transit Coalition, which is campaigning for the referendum, declined to say how many voters it anticipates, only that it thinks it will be a record number for a May election.

"We expect a record number of voters to come out for this. It's important Nashville has raised its voice and made it clear they wanted something to happen (on transit), and now is their chance," transit coalition spokeswoman Kelly Brockman said.

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But Jeff Eller, a spokesman for NoTax4Tracks, the lead group fighting approval of the plan, projects that only around 40,000 people will vote — although he cautioned that it's just a guess.

He predicted "some voter apathy" after last month's resignation of Mayor Megan Barry, who had been the most visible proponent of the plan. Mayor David Briley is now pushing the project.

"I don't know what to expect," Eller said. "Clearly, interest in the transit issue could increase turnout. But we're having a hard time judging it.

"Barry versus Briley — that whole dynamic: How does it affect voter enthusiasm?" Eller said. "Briley is not an overly compelling figure like she was. Can he coalesce his base and keep the turnout numbers where they need them or is there falloff?"

Transit referendum alters other races

Candidates in the other races up for grabs on May 1 also have braced for a higher-than-usual turnout, realizing it will have an effect on their contests.

Richard Exton, a candidate for register of deeds, said he initially expected about 75,000 to go to the polls, but he now believes it will be lower because of the confusion and high number of undecided voters on the transit question.

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"Our fear is that will mean they may not come out as they probably should," Exton said.

In a move not typically seen in local races like register of deeds, Exton on Friday went on television with an advertisement, paying $75,000 to run the 30-second spots.

"Part of the reason we went up on television is because we just don't know who the electorate is," he said. "We're trying to spread as broad of net as we can."

Both sides in the transit debate have been engaging in traditional get-out-the-vote efforts this past week, including volunteer and paid phone-banking and canvassing to go along with direct mail and TV ads.

Voters can expect signs and representatives at the polls from referendum supporters and critics.

The transit coalition has a partnership with the ride-hailing program Lyft, in which it will offer rides to polls that are 50 percent typical fares through early voting, which ends April 26. The coalition also is promoting a phone app called Transit, which will offer users interactive maps and other tools to learn about voting sites and the ballot.

Because of the greater interest in this year's local primary election, the election commission is beginning early voting by opening all sites, as opposed to only the downtown location, as is the tradition.

Early voting sites

• Belle Meade City Hall: 4705 Harding Pike, Nashville 37205

• Bellevue Library: 720 Baugh Road, Nashville 37221

• Bordeaux Library: 4000 Clarksville Pike, Nashville 37218

• Casa Azafrán Community Center: 2195 Nolensville Pike, Nashville 37211

• Edmondson Pike Library: 5501 Edmondson Pike, Nashville 37211

• Goodlettsville Community Center: 200 Memorial Drive, Goodlettsville 37072

• Green Hills Library: 3701 Benham Ave., Nashville 37215

• Hermitage Library: 3700 James Kay Lane, Hermitage, 37076

• Howard Office Building, Sonny West Auditorium: 700 Second Ave. S., Nashville 37210

• Madison Station Fifty Forward: 301 Madison St., Madison 37115

• Southeast Library: 5260 Hickory Hollow Parkway, Antioch 37013

Reach Joey Garrison at jgarrison@tennessean.com or 615-259-8236 and on Twitter @joeygarrison.