Gear and clutch problems in the Osprey have killed more than a dozen troops in recent years, leading to groundings of the tilt-rotor aircraft.
Democratic lawmakers vow to block bills, slow nominations and hold their own hearings to protest planned cuts to veterans programs.
The U.S. continues to strike Houthi militants, who have targeted Israel and American ships in the region.
President Donald Trump has said Caine wore the hat and expressed political support during a 2018 visit to Iraq.
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