Predicting about 750,000 of the roughly 4 million eligible voters will cast a ballot in the Democratic and Republican primaries for an open U.S. Senate seat, Secretary of State William Galvin said he hopes a last minute surge of excitement and get-out-the-vote efforts will foster a bigger turnout.

"I'd very much like to be wrong," Galvin told reporters Monday at a pre-election briefing, saying the similarity of the two Democratic candidates, the April 15 bombings at the Boston Marathon, and the holding of a springtime election resulted in a lack of enthusiasm.

"This primary has not had the same momentum. Obviously the tragic events of April 15 serve to cause people to think about other things other than politics," Galvin said, noting the bombings had affected media coverage and caused a debate to be canceled.

After years of stability among the state's two U.S. senators, Massachusetts is now heading into its second special election for the office in three years, and Galvin said voters appear less enthusiastic than in 2009.

Contrasting Tuesday's primary to fill the seat of Secretary of State John Kerry with a special Senate primary in 2009 to fill the seat of the late U.S. Sen. Ted Kennedy, Galvin said the Democratic field is less diverse, while there is a more competitive three-way race on the Republican side.

"Here in the Democratic side, you have two people, even though there may be some policy differences, who are remarkably similar in terms of background and outlook," Galvin said, referring to Congressmen Stephen Lynch and Ed Markey. The Republican field, comprising businessman Gabriel Gomez, Rep. Dan Winslow (R-Norfolk) and former U.S. Attorney Michael Sullivan is "more diverse" this year, Galvin said.

Galvin said the 550,000 he predicts to vote in the Democratic primary is "generous" and a decline from the 669,000 Democratic voters in 2009, when Attorney General Martha Coakley won the primary. Galvin predicted a bump in participation from Republicans compared to 2009, when Scott Brown won, saying he expected participation to increase from an "extraordinarily low" 165,000 in 2009 to 200,000 on Tuesday.

Party members may only vote in their party primary; unenrolled voters may vote in either party primary.

Galvin said Markey appears to have inherited much of Sen. Elizabeth Warren's get-out-the-vote operation and Lynch has a number of union members volunteering for the campaign, which he said he hopes will increase turnout Tuesday. With an absentee ballot application deadline of noon Monday, Galvin said there had been a 20 percent drop in absentee ballot requests from people voting in the Democratic primary.

Eighty-six municipalities have "piggybacked" local municipal elections with the special Senate primary, and Boston has a special primary to replace former Sen. Jack Hart (D-South Boston). That race will "have a feeder effect," boosting turnout in Suffolk County from the 70,000 who voted in 2009, Galvin said.

Voters may be greeted in Boston by candidates running in a race that's not on the ballot Tuesday. By coincidence, papers for the open Boston mayor's race, will be available at 9 a.m. Tuesday, and Galvin said the high number of candidates for that office will be kept 150 feet from polling locations.

"Voters shouldn't have to run a gauntlet of candidates," Galvin said.

Asked about the number of people running for the seat - which Mayor Tom Menino said he will not seek again after two decades in power - Galvin quipped, "It's possible in some places there will be more people running for mayor than there are voting."

Spring weather will make it easier for people to get to the polls, but it is also incongruous with the fall weather that usually accompanies elections, Galvin said. He said the special election would cost the state $5.2 million, and disputed Auditor Suzanne Bump's assertion that the special election is an unfunded mandate on the cities and towns that administer the voting.

"It seems to me unreasonable at this point to say when you're giving people the opportunity to vote on the United States Senate it's a burden on cities and towns and they need to be compensated dollar-for-dollar on it," said Galvin, who said the state is prepared for everyone to vote and provides some funding to local election administrators. It is up to the Legislature to decide if town clerks and city election commissions should receive a reimbursement, Galvin said.

Galvin also reported on what he said were 5,000 voters who left the Democratic Party rolls with 700 joining the Republican Party and 4,300 registering unenrolled. Galvin speculated that those voters were leaving the party "at the behest of so-called right to life groups" to vote for Sullivan in the GOP primary.

While early indications show a low turnout, Galvin said it is up to candidates to motivate voters and said he hopes they are successful in increasing the number of people voting.

"We have a lot of eligibility out there," Galvin said, noting that about 4 million people are eligible to vote and the state tries to encourage more electoral participation. "Motivation is up to the candidates and the citizens to come to a conclusion. I hope that they will be motivated, and I'm hoping at the last minute people will be focused on this."

Galvin said the winner would probably be known by around 10 p.m. and said a race close enough to trigger a recount is unlikely. The last statewide recount was in 1994, Galvin said, when he was first elected. That year, Arthur Chase edged Republican Peter Forman in the primary for secretary of state.

"I was very grateful for the fact that Republicans couldn't make their minds up," Galvin said.

Polls are open Tuesday from 7 a.m. until 8 p.m.