WASHINGTON—American Airlines and JetBlue Airways Corp asked a U.S. judge on Monday to dismiss an antitrust lawsuit filed by the Justice Department and six states over the company’s “Northeast Alliance” partnership. The suit filed in September alleges the deal would lead to higher fares in busy northeastern U.S. airports and seeks to unwind the alliance. The airlines said Monday the alliance has been in place for nine months but the suit does “not allege that it has caused a single higher price, any reduction in quality or the slightest reduction in output.” The Justice Department declined to comment on Monday. The government and airlines on Friday backed a proposed trial date of September 2022 but that schedule has not yet been approved by a U.S. judge. The partnership was announced in July 2020 and approved by the U.S. Transportation Department (DOT) shortly before the end of the Trump administration. The …