Trump and Qatari Emir Tamim bin Hamad Al Thani signed several agreements earlier on Wednesday, with the first being a Boeing order from Qatar Airways.

“We cannot accept a $400 million ‘gift’ from jihadists in suits,” Laura Loomer, a far-right activist whose advice the president has occasionally heeded, wrote on social media.

Democrats on the Senate Foreign Relations Committee said Trump’s acceptance of the plane would create “a clear conflict of interest, raises serious national security questions, invites foreign influe...