NEWARK — Democrat David Bentz and Republican Eileen O’Shaughnessy-Coleman are squaring off in the 18th Representative District special election, set to take place sometime in the first half of September.

The 18th District seat, which covers the Newark area, has been held by Democrat Rep. Michael Barbieri since 2008, but he resigned Monday to take a job with the new Department of Health and Social Services.

The House Democratic caucus announced in July Mr. Barbieri had been named the new director of the Division of Substance Abuse and Mental Health and was stepping down from his elected post.

Later that month, Mr. Bentz and Ms. O’Shaughnessy-Coleman were selected by the county parties to run for the seat.

On Thursday, the Democratic Party released a statement criticizing the GOP.

Michael Barbieri

“Republican Party bosses hand-picked their candidate for the upcoming special election without local input and are lining up to campaign for her, just like they expect her to line up with them in the House if she’s elected,” Majority Leader Rep. Valerie Longhurst, D-Bear, said.

“Republicans expect their candidate to toe the party line in voting blindly against initiatives that benefit all Delawareans, as all House Republicans did in voting lock-step this year against funding for much-needed infrastructure repairs and improvements. The state GOP desperately wants another vote in the House so they can push their strategy of delay, obstruction and manufactured crisis.”

In response, the Republican Party fired back.

“The Democratic statement is as ludicrous as it is hypocritical,” Republican Party Executive Director John Fluharty said. “David Bentz is a paid House staff person for the outgoing state representative and was selected by his party bosses despite living in the district for just two years. No wonder they want to smear Eileen O’Shaughnessy-Coleman, a woman who has nearly 25 years’ community service.”

Members of both parties have alleged the opposing side’s process was not open. The Democrats notified registered party members in the district and held a special meeting where the committee voted on the interested individuals, unanimously choosing Mr. Bentz. The Western New Castle Republican Committee interviewed people who applied and then selected Ms. O’Shaughnessy-Coleman, who members felt was the best candidate.

The now-vacant seat carries extra impact because a Republican win can break the Democratic supermajority in the House. Currently, Democrats have 27 of the 41 seats, giving them slightly more than 60 percent. That means they can pass tax bills in the chamber without minority support.

If Ms. O’Shaughnessy-Coleman is victorious, both the House and Senate will no longer have a supermajority. Senate Democrats lost their 60-percent threshold in November’s general election.

District 18 is heavily Democratic, but prior to Mr. Barbieri’s 2008 victory, Republican Terry Spence held the seat for 26 years. At least by registration numbers, the makeup heavily favors Mr. Bentz: there are about 8,600 Democrats to 2,900 Republicans and 3,600 independents in the district.

The election date will be selected soon by Speaker of the House Pete Schwartzkopf, D-Rehoboth Beach.