Independent voters in the state, who can vote in either the Democratic or Republican primary, make up 50.3 percent of the state's 4 million voters. Registered Democrats account for 36.8 percent and Republicans make up 12.1 percent. Smaller parties make up less than 1 percent.
Galvin said most independent voters would probably cast ballots in the Democratic race.
"I think they are more likely to break to the Democratic side, because most independent voters in Massachusetts have historically Democratic roots," he said. "But I think there will be a healthy independent voters on the Republican side."
Republican Mary Jordan said she didn't ultimately decide to vote for Romney until she entered the voting booth.
"I think he's the least unlikeable. I really didn't like any of them," said Jordan, a 43-year-old teacher's aide from Topsfield.
Monica Crowley, 40, said she couldn't vote Democrat because she doesn't like talk show host Oprah Winfrey, who supports Obama, and she opposed Clinton's health care plan.
Ultimately, she reasoned that McCain, a staunch supporter of the Iraq war, would actually be most likely to end the conflict because his son, Jimmy, is serving in Iraq.
"I can't believe after what he's been through and having (a son) there that he will allow it to continue," she said.
In Randolph, an elderly man drove over a sidewalk and struck an 8-year-old girl outside an elementary school being used as a polling place. Authorities say the man lost control of his SUV and drove through a crowd of kids, ultimately pinning the girl against the brick building. The girl was taken to a Boston hospital and her condition was not immediately known. Officials said voting was not disrupted for the two precincts that use the building.
Polls across the state are open until 8 p.m.