"I can give you a winning scenario for every one of those candidates," said Larry Ceisler, a political consultant who has ties to all four candidates but who said he wasn't backing one over the others. "As we get closer, it's going to tighten up even more," he said.
The candidates are scheduled to debate at 7 p.m. Tuesday at the Philadelphia Portuguese Club, 2019 Rhawn St.
Over the last six months, Margolies' fund-raising has not kept up with her spending, and she has been battered by negative reports about her charity, her ex-husband's financial crimes, and her campaign's reliance on the Clintons (Chelsea Clinton is her daughter-in-law).
Last week, that criticism was amplified when Leach filed a complaint with the Federal Election Commission, alleging that Margolies misused money designated for the general election.
Six months ago, "I would have said [Margolies] is a 2-to-1 or 3-to-1 favorite," said Neil Oxman, a Democratic consultant who is not affiliated with the race. "Given what's happened with her finances, I think she's no longer the front-runner."
Margolies' campaign has denied breaking any finance laws. Her campaign manager, Ken Smukler, said internal polling indicated that she still had a strong lead in the race.
But with three weeks until the May 20 primary, independent observers said it was anyone's game.
"Each of the four candidates really has a strong argument that they're best equipped to win the primary," said Mark Nevins, a consultant with Dover Strategy Group who said he was "officially agnostic" on the race. He said the result would come down to "who has the most compelling message and the resources to get that message out on TV and by mail aggressively."
Leach and Boyle are running TV ads, and Arkoosh has independent radio ads and direct mail going out.
Margolies' campaign on Monday will open a Northeast Philadelphia office. But as of late March, it had spent no money on advertising or direct voter outreach, according to FEC reports. Her first TV ad is set to air this week.
Geography vs. gender
The nominee will square off in the fall against the winner of the GOP race between retired U.S. Air Force Col. Beverly Plosa-Bowser and businessman Carson "Dee" Adcock.
Any Democrat is likely to have the advantage. The 13th District is reliably Democratic - roughly 60 percent - and the population is split evenly between Northeast Philadelphia and southern Montgomery County.
Margolies, Leach, and Arkoosh hail from Montgomery County - which could give Boyle an edge.
Oxman, a consultant with the Campaign Group in Philadelphia who isn't supporting any candidate, likes Boyle's odds.
He and others praised Boyle's ground game and said his support among trade unions and Philadelphia ward leaders would pay off on Election Day.
Boyle is "certainly going to win the city," Oxman said. "The question is, will he win the city by a large enough margin that whoever wins the suburbs will not matter?"
Ceisler and Nevins agreed the suburban split could be a deciding factor in Boyle's favor.
Margolies and Arkoosh have been campaigning hard on the importance of electing a woman to replace Schwartz, currently the only woman in Pennsylvania's congressional delegation.
Margolies has spent 20 years leading a charity that promotes women's leadership, and she casts herself as the leading proponent for women.
Arkoosh, who lobbied around the country for the Affordable Care Act and its women's health provisions, in the spring received a high-profile endorsement from the Women's Campaign Fund. Last week, the Women's Campaign Fund named Arkoosh one of its 40 "game changers" in U.S. politics.
Emily's List, a leading national women's political group, has yet to issue an endorsement in the 13th District.
Nevins said gender could sway some voters and noted that women tend to vote in "slightly larger numbers in elections like this." But because there are two women in the race, he said, "I'm not sure there's an obvious winner."
Last week, the campaign took a negative turn when Leach accused Margolies of deliberately misusing campaign money.
Margolies' campaign has denied the allegations and hired a former FEC commissioner to represent her in the case.
The candidates have also turned up the heat to highlight their few policy differences.
Arkoosh has been attacking Margolies' 1994 plan to cut Social Security benefits and raise the retirement age. "Marjorie Margolies tried to slash our Social Security benefits," one of her campaign mailers declared.
In an e-mail Friday, Margolies called those mailers "intellectually dishonest" and has said she does not agree to benefit cuts or raising the retirement age.
"My opponents continue to attempt to scare seniors by citing positions I took over two decades ago as though they are current," Margolies wrote.
She has criticized Leach for supporting positions she says are politically untenable, among them legalizing marijuana and shifting to a single-payer health-care system.
Leach, Arkoosh, and Margolies have criticized Boyle for voting to tighten regulations on abortion providers. Five of the state's 22 clinics closed after the bill passed.
Boyle said he supported women's right to choose but defended the vote.
"I think it's a reasonable step to say there need to be reasonable health requirements and inspections," he said Friday.