Washington, United States — The leader of the Democratic minority in the US Senate introduced legislation Monday that would block President Donald Trump from using a luxury plane offered by Qatar as the official Air Force One.

Chuck Schumer's Presidential Airlift Security Act would prohibit the Pentagon from using taxpayer funds to retrofit any plane previously owned by a foreign government for use as the presidential plane.

The bill marked the latest in a series of protests by Democrats over Trump's acceptance of the $400 million 747-8 jumbo jet, offered free-of-charge by the Gulf state's royal family.

The proposal has raised huge constitutional and ethical questions -- as well as security concerns about using an aircraft donated by a foreign power for use as the ultra-sensitive Air Force One.

Trump has denied there are any ethical issues involved with accepting the plane, saying it would be "stupid" for the US government not to take the aircraft.

"Donald Trump has shown time and again he will sell out the American people and the presidency if it means filling his own pockets," Schumer said in a statement. 

"Not only would it take billions of taxpayer dollars to even attempt to retrofit and secure this plane, but there's absolutely no amount of modifications that can guarantee it will be secure."

Although several Republicans have voiced concerns about the proposed gift, Senate Majority Leader John Thune -- a Trump loyalist -- is not obliged to bring the bill to the floor of Congress's upper chamber.

But Schumer plans to force a vote by offering it as an amendment to spending bills that Republicans will have to pass later in the year.

The US Constitution prohibits government officials from accepting gifts "from any King, Prince or foreign State," in a section known as the Emoluments Clause.

Schumer announced last week that he was planning to hold up all Justice Department political appointees over the gift, while Senator Chris Murphy pledged to force votes to block weapons sales to Qatar.

Other Democratic senators have sought to advance resolutions of disapproval and have discussed legal action to enforce the Emoluments Clause.

Some Republicans have also appeared queasy over the offer, mainly raising security concerns or objecting to the high costs of modifying a craft that will only be used during Trump's term, before being moved to his presidential library. 

Trump has long been unhappy with the current Air Force One jets -- two highly customized Boeing 747-200B series aircraft that entered service in 1990 under president George H.W. Bush.