Secretary of the Commonwealth William Galvin estimates that around 700,000 Massachusetts voters will cast ballots in Tuesday's primary, as experts warn that the post-Labor Day date could depress turnout.

"Normally, turnout in primaries is terrible," said Erin O'Brien, associate professor of political science at UMass Boston. "Given the timing of this primary, turnout will be abhorrent."

Galvin's estimates would place turnout at around 15 percent of the 4.49 million people registered to vote in the 2018 state primary.

Galvin estimates that around 450,000 to 500,000 people will vote in the Democratic primary, and around 200,000 voters will cast Republican ballots.

Statewide, 33 percent of the electorate is Democratic, 10 percent is Republican and 55 percent is unenrolled.

Unenrolled voters can choose which primary to vote in.

Four years ago, when there were active Democratic races for governor, treasurer and attorney general and a major congressional race in the 6th District -- where Seth Moulton defeated incumbent John Tierney -- 556,000 voters cast ballots in the Democratic primary. Two years ago, when the only significant races were legislative and county races, 313,000 Democrats voted.

Dot size in the chart above represents the number of candidates appearing on the ballot. Click or tap a dot to show more information, including the primary winner.

Chart by Greg Saulmon / The Republican

Source: Massachusetts Secretary of the Commonwealth

In this year's primary, Democrats will have competitive statewide races for governor, lieutenant governor and secretary of state.

There is a 10-way race for the 3rd District congressional seat that U.S. Rep. Niki Tsongas is leaving and a tight race in the 7th District where Boston City Councilor Ayanna Pressley is challenging U.S. Rep. Michael Capuano. Several other incumbent members of Congress are facing Democratic challengers, including 1st District Rep. Richard Neal.

In Western Massachusetts, a number of legislative races could drive up turnout, including a race for the Senate seat vacated by Sen. Stan Rosenberg. There are also contests for open House seats that were held by Rep. Peter Kocot, who died, and Reps. John Scibak and Stephen Kulik, who are retiring.

The Boston area has several competitive House races and an active Suffolk District Attorney race. Galvin said Lawrence and Lowell could also have high turnout due to legislative races. But much of Norfolk, Plymouth and Worcester counties have very few local contests.

On the Republican side, there are statewide primaries for governor, between incumbent Gov. Charlie Baker and challenger Scott Lively, and for U.S. Senator, where three Republicans are vying to take on U.S. Sen. Elizabeth Warren. But there are very few other competitive races.

"There's no local Republican contests. It's very flat," Galvin said. "The question on the Republican side to what extent the U.S. Senate contest and Gov. Baker will drag them out."

In 2014, 160,000 people cast ballots in the Republican primary, which Baker won in a landslide.

In 2010, 241,000 people voted in the Republican primary. The statewide races were not seriously contested that year, but there were several competitive congressional primaries.

But Galvin said predictions are more difficult than usual this year. He typically makes turnout predictions based on the number of absentee ballots cast.

Both four and eight years ago, just over 26,000 people cast absentee ballots in the Democratic primary. As of this Wednesday, 35,000 had cast absentee ballots.

Galvin said he does not think this forecasts a spike in turnout, but rather reflects the fact that more people will be out of town the day after Labor Day. And after the debut of early voting in 2016, some voters think they can vote early this year, even though the Legislature did not authorize early voting for the primary.

Galvin set the primary for the Tuesday after the Labor Day holiday to meet state and federal requirements, while avoiding the Jewish holidays and the first day class for Boston schools.

His primary opponent, Boston City Councilor Josh Zakim, has said choosing that date was an attempt to suppress turnout, which could help incumbents.

While acknowledging that the date would not have been his first choice, Galvin said he does not think the Sept. 4 primary will lower turnout that much. He noted that many schools already started this week -- so parents are getting back into routines and families are less likely to be on vacation.

Galvin said the large amount of advertising in areas like Boston has made it hard for people to ignore the election.

Galvin noted that a special election to replace former U.S. Rep. Marty Meehan was held the day after Labor Day in 2007, and turnout was high. In that race, 106,000 voters cast ballots, and Tsongas won with 51 percent.

But some experts say voting the day after Labor Day cannot help but depress turnout.

"It's always hard for primaries, but every factor we could do to make it even harder for folks to turn out, we've done by scheduling it the day after Labor Day," O'Brien said.

O'Brien said the people paying attention to primaries are normally strong partisans, but "even strong partisans like Labor Day."

She said people often start paying attention to politics only once school is back in session. "I'd predict people are going to start paying attention and realize the primary already happened," O'Brien said.

O'Brien said states with summer primaries have had turnout around 10 percent, and she thinks Massachusetts' turnout could be in the single digits.

She said incumbents, like Galvin, are likely to benefit from a low-turnout race, because incumbents have greater name recognition and a stronger network of supporters in the district.

"People turning out know the incumbent's name," O'Brien said. "It's hard for the challenger to break through pre-Labor Day."

UMass Amherst political science professor Ray La Raja said excitement by political activists is likely to be counteracted by people sending their children back to school, returning to work and coming home from vacation. La Raja said people who pay little attention to politics are unlikely to show up at the polls the day after Labor Day.

"It's not a great time for holding an election," he said.

La Raja said he is teaching an undergraduate political science class on Tuesday. He plans to ask his students how many of them know there is an election that day.

"I'll see how many hands go up," La Raja said. "I guarantee it will be less than a majority."