WASHINGTON -- Bipartisan opposition is emerging in the Senate to a plan by House lawmakers to spend $550 million for additional passenger jets for senior government officials.

The resistance to buying eight Gulfstream and Boeing planes comes as members of both chambers of Congress embark on the busiest month of the year for official overseas travel. The plan to upgrade the fleet of government jets, which was included in a broader defense-funding bill, has also sparked criticism from the Pentagon, which has said it doesn't...