WASHINGTON — Continuing his push into high-profile congressional races, former Vice President Joe Biden endorsed two Democratic candidates in the Philadelphia suburbs Thursday: Chrissy Houlahan of Chester County and Andy Kim from Burlington County.

Biden's backing arrived on the heels of his support for Democrat Conor Lamb in a nationally watched special election in Southwestern Pennsylvania, and as Democrats attempt to build on their momentum in critical suburban battlegrounds that could determine control of the House this fall.

While Biden's endorsements of fellow Democrats are not surprising, they also come as he stirs speculation about his future as a possible 2020 presidential candidate. Biden, 75, has cast himself as a son of Scranton who can reach the working-class swing voters who have fled his party. He is also scheduled to visit Capitol Hill next week to speak with the Democratic Blue Collar Caucus, said Rep. Brendan Boyle (D., Pa.), the group's co-chairman.

Houlahan is aiming to oust Rep. Ryan Costello in Pennsylvania's Sixth District, while Kim is challenging Rep. Tom MacArthur in New Jersey's Third, which includes much of Burlington and Ocean Counties. Both are considered among the potentially most competitive races in the country. There has been speculation that Costello might choose not to run again.

Biden cited both Democrats' ties to national security, much as he did in praising Lamb's service in the Marines.

"Chrissy Houlahan is an Air Force veteran, business leader, and former teacher, and she is precisely the type of public servant we need serving in Congress," Biden said in a news release issued by the Houlahan campaign.

In a release from the Kim camp, Biden said, "Andy's family story embodies the American story. His parents grew up poor and worked their way to America for a better life. … Their son worked hard and decided to devote his life to service." Kim was a national security adviser in the Obama administration.

Republicans anticipating difficult campaigns have attacked Houlahan's previous business practices and questioned Kim's independence from his party's left wing.

"It's disappointing that former Vice President Biden would support greedy Chrissy Houlahan," emailed Costello campaign spokeswoman Andrea Bozek, who reiterated accusations that workers were mistreated while the Democrat helped lead the T-shirt company And1.  "He knows Pennsylvania families deserve better."

A spokesman for MacArthur said that like Biden, the GOP representative has won respect for an "ability to work in a bipartisan way to solve problems."