NORTH ANDOVER -- A prosecutor. A politician. A businessman. And a citizen activist.

The biographies of the four Democrats vying to replace Edward Kennedy in the U.S. Senate could determine whom voters choose in a primary election that features a field of candidates with remarkably similar policy positions on everything from the war in Afghanistan to health care.

"I wish I was running against George Bush and the differences were painfully obvious," joked U.S. Rep. Michael Capuano, acknowledging that little separates him from his opponents when it comes to the issues.

The four Democratic candidates for U.S. Senate -- Capuano, Attorney General Martha Coakley, Celtics co-owner Stephen Pagliuca and City Year co-founder Alan

Khazei -- participated in a candidate forum Sunday afternoon at Merrimack College, the first such event of the young campaign in which all four candidates submitted to extended questioning.

Though the forum was not quite a debate, each took questions for 30 minutes, explaining why voters should choose them in the upcoming Dec. 8 primary. The event was sponsored by the Greater Lowell Area Democrats, the North Andover Democratic Town Committee and a host of other regional organizations.

"Of all the candidates, I have actually been doing the job for the past 11 years in the House, and I have a long record," Capuano said.

The record includes some votes he would like to take back, such as one in 2000 as a member of Financial

Services Committee to deregulate the commodity-futures market and also for pushing as hard as he did to make home mortgages available to as many people as possible.

Still, in his straight-forward, self-deprecating style, Capuano said he is proud of his overall record that includes voting against the Patriot Act and speaking out against the war in Iraq. He said he believes homeownership should still be a priority with sensible regulations.

The other three Democrats in the field, however,

are not willing to concede the mantle of most experienced to Capuano just because he is already a member of Congress.

All four candidates agreed on the need for a "robust" public health-care option, expressed skepticism about boosting troop levels in Afghanistan without a clear mission, and said they would fight to repeal the Defense of Marriage Act, which limits how state and federal agencies recognize same-sex marriages.

It's the other intangibles that define "leadership," however, that could prove to be the difference.

Coakley focused in on her "two and half decades" of public service from the time she was a Middlesex County prosecutor and district attorney, to the past three years she has served as attorney general.

Highlighting her accomplishments as attorney general, Coakley pointed to lawsuits and legislation she has filed to crack down on predatory mortgage lending, her work to protect seniors and children from cyber crime, her efforts to go after big pharmaceutical and insurance companies that have driven up the cost of health care, and the federal lawsuit she filed to strike down DOMA.

The early questioning from moderator Renée Loth, a Boston Globe columnist and former editorial page editor, focused on the one glaring weakness in Coakley's resume -- foreign policy.

Coakley tip-toed her way through an answer on the growing threat of a nuclear Pakistan, but offered a more direct answer on whether President Barack Obama should send more troops to Afghanistan.

"I think we need to have a clear mission about what we are doing in Afghanistan. I don't think the case has been made yet that the mission is clear and that more troops will help us accomplish that," Coakley said.

Pagliuca got the afternoon started, talking about his family and humble upbringing that drives him to make sure "the American dream" remains attainable for families in Massachusetts.

"I spent 25 years on the ground building businesses. I'm the only candidate with that experience," Pagliuca said.

Pagliuca said he is hoping his business background investing in startups as an executive at Bain Capital will convince voters he has the chops to create jobs and get the economy moving again. He said making smart investments that will help the life-science and clean-technology sectors grow in Massachusetts will not only make the state a leader in those fields but bring manufacturing jobs back to cities like Lowell and Lawrence.

He also said tighter financial regulations and stricter enforcement over Wall Street and bank lenders is necessary to prevent another economic collapse. He said potential homebuyers should have to meet strict deposit and income tests to qualify for a mortgage, and lenders who don't follow these rules should be closed.

"This is why we need people in Congress who understand business," Pagliuca said.

Pagliuca also admitted that not voting in several presidential primaries was a "big mistake," and said past support for Republican candidates like Mitt Romney were "out of friendship" and not philosophy.

Khazei, co-founder of the community-service organization City Year, said he is not about "big government" or "big business," but rather "big citizenship."

He said he is the only real reformer in the race who, despite lack of name-recognition or personal wealth, has refused to take donations from lobbyists or PACs.

"We all voted for change last November, and we're not getting it and that's because of Congress," Khazei said.

A bundle of energy, Khazei painted himself as the only candidate who knows how to build "a movement" for real reform and will use those skills to fight for a health-care mandate and strong public option, for better teacher pay and innovation in education, and for civil rights.