Brendan Boyle, flanked by his brother Kevin, speaks at a campaign party Tuesday in Philadelphia after winning the Democratic primary in the 13th District, virtually assuring him of becoming the successor to U.S. Rep. Allyson Schwartz.

PHILADELPHIA — A young state representative from Philadelphia beat three rivals — including a former congresswoman who is now Chelsea Clinton’s mother-in-law — to win the Democratic nomination Tuesday for an open seat in the U.S. House.

Early returns showed Brendan Boyle winning the Democratic primary in Pennsylvania’s 13th District, virtually assuring him of becoming the successor to U.S. Rep. Allyson Schwartz, a fifth-term Democrat who unsuccessfully sought the Democratic nod for governor. The district straddling Philadelphia and Montgomery County is heavily tilted toward Democrats.

Boyle, the only candidate from Philadelphia, benefited from a unique power base: he and his brother, Kevin, are both state representatives whose legislative districts lie within the 13th District. A political committee made up of building-trades unions also aired TV ads supporting his bid.

Marjorie Margolies, who got fundraising help from the Clintons, was looking to recapture the 13th District seat she held from 1993 to 1995 before two rounds of redistricting made the district more reliably Democratic. Voters denied her a second term after she cast a crucial vote for then-President Bill Clinton’s budget plan that increased taxes on the wealthy.

The other candidates were state Sen. Daylin Leach, a champion of the party’s progressive wing; and Dr. Valerie Arkoosh, an obstetric anesthesiologist. Arkoosh topped her rivals in fundraising by taking in more than $1.9 million, even though she was the only candidate in the field with no experience running for elected office.

The seat, one of two open U.S. House seats in Pennsylvania this year, is being vacated in January by Schwartz.

On the Republican side, businessman Carson Dee Adcock defeated retired Air Force Col. Beverly Plosa-Bowser. Adcock sought the seat in 2010 but lost to Schwartz.

The district takes in parts of Philadelphia and Montgomery County. Democrats account for nearly 60 percent of the voters, so a party primary win is likely to ensure success in the Nov. 4 election.

The only other open seat is in the 6th District, which takes in parts of four counties in southeastern Pennsylvania. Six-term Republican Rep. Jim Gerlach is stepping down, and neither the Republican nominee nor the Democrat was opposed in the primary.

Both the former president and former Secretary of State Hillary Clinton appeared at fundraisers for Margolies, whose son, Marc Mezvinsky, married Chelsea Clinton in 2010.