WORCESTER— While it’s sometimes hard to tell one candidate’s positions from another’s, voters in the 16th Worcester district will have a clear choice in the Democratic primary Sept. 19.

There are numerous differences between John P. Fresolo, who has held the state representative’s job for four two-year terms, and Melissa J. Murgo, challenging him for the second time. The 27-year-old challenger used to be a legislative aide to the 41-year-old legislator, but since then they haven’t even been able to agree on whether she quit or he fired her.

There are differences in background between them, such as education. Mr. Fresolo has a two-year business degree from the former Central New England College; Ms. Murgo has a bachelor’s degree from Boston College and a master’s degree from Framingham State College.

If endorsements are an indication, Ms. Murgo is the more liberal. Mr. Fresolo has a host of endorsements from public safety employees and other labor unions, including Teamsters Local 170, the Massachusetts Nurses Association and the Massachusetts Teachers Association, as well as from the Gun Owners’ Action League.

Endorsing Ms. Murgo are pro-choice groups, including NARAL Pro-Choice Massachusetts, Planned Parenthood Advocacy Fund and National Organization for Women; environmental groups — the Sierra Club and Clean Water Action — as well as a coalition called Mass Alliance, which includes, curiously, the Massachusetts Teachers Association.

The incumbent turns away questions about how he differs from Ms. Murgo, saying he wants to run a “positive, goal-oriented campaign.” He touts what he calls an unprecedented amount of state money coming into the district during the past two years.

Still, differences emerge. They have different views of the Massachusetts Comprehensive Assessment System, even though both call education one of their top priorities.

Ms. Murgo said MCAS test scores should not prevent an otherwise qualified student from graduating. She agreed with a voter who approached her recently saying there are capable students who don’t test well.

And, while Mr. Fresolo said he knows there are bugs in the MCAS testing that need to be corrected, students must pass MCAS or something like it. “We need to have some program in place to be sure that our children are prepared to go on to college, if that’s what they decide do.”

Ms. Murgo said she favors allowing same-sex marriage, calling it a civil right as well as a revenue generator, attracting wedding ceremonies and new residents to the state.

“I personally support traditional marriage,” Mr. Fresolo said. “And I supported the amendment to put that on the ballot and allow the voters of Massachusetts to vote on it.”

His challenger said she would have voted differently from Mr. Fresolo on a number of occasions. She opposes capital punishment, favors paid family leave and would have voted to tie future increases in the state’s minimum wage to cost-of-living increases.

While she would have voted to give a company such as Raytheon tax breaks, in return for expansion or keeping jobs in the state — as Mr. Fresolo did — “I would have proposed legislation to retract the tax benefits that they were previously granted” when the company failed to produce on its promises, she said.

She also said that she would not have accepted the pay raise legislators voted themselves in the midst of a financial crisis in 2003, as Mr. Fresolo did; she still would not accept it, she said, because the state is not yet out of its difficulties.

The greatest difference, of course, is that Mr. Fresolo is the incumbent. That allows him to boast of state money that has come into the district in the past two years, including, he said, $180 million of the $300 million Route 146 project, and more than $200 million in state aid to the city budget.

Beyond $650,000 that the entire city gets for community policing, Mr. Fresolo said, $95,000 is earmarked for projects in the 16th Worcester District, $1.6 million went to the Blackstone Gateway Visitors Center in the past year, and $1.18 million has come in to finance conversion of the old David Burwick Furniture Building into housing — a project that had stalled before its location was redistricted into the 16th District.

Mr. Fresolo said he also helped secure $500,000 in state funds when federal dollars were insufficient to complete 35 units of housing for the elderly at Ascension Church.

According to Ms. Murgo, Mr. Fresolo’s efforts have not always been sufficient. She says it took the intervention of U.S. Rep. James P. McGovern, D-Worcester, and the rest of the state legislative delegates from Worcester to get the Route 146 project back on track a few years ago. She also said Mr. Fresolo has failed to put enough effort into attracting biotech companies to the city through infrastructure investments and tax incentives, and that more effort needs to be expended to secure additional commuter rail for the city.

The district includes Precincts 1, 2, 4 and 5, of Ward 5; all precincts of Ward 6; and Precincts 1 and 5 of Ward 8, all in Worcester.