Adds Qatari prime minister statement, paragraph 3

By Richard Cowan and Patricia Zengerle

WASHINGTON, May 14 (Reuters) - Multiple congressional Republicans raised concerns about President Donald Trump's desire to accept a $400 million airplane from Qatar, as rival Democrats slammed the plan as an illegal foreign donation and national security threat.

Trump said on Monday that it would be "stupid" for him to refuse Qatar's offer of the Boeing BA.N 747-8 airplane, which would be used as U.S. "Air Force One," the jet American presidents travel on when they fly around the globe.

Qatari Prime Minister and Foreign Minister Sheikh Mohammed bin Abdulrahman Al Thani told CNN on Wednesday that the offer was "a very simple government-to-government dealing," not a personal gift to Trump, and that "of course" it would be withdrawn if the transaction were deemed illegal.

The aircraft eventually would be donated to Trump's presidential library foundation and used by him after leaving office.

"There will be plenty of scrutiny," Republican Senate Majority Leader John Thune of South Dakota told reporters. "There are lots and lots of issues around that, that I think will attract very serious questions."

Trump's interest in the plane became public as he made a high-profile Middle East trip that included an announcement by state carrier Qatar Airways that it would purchase up to 210 widebody jets from Boeing.

Experts have said it would take years and hundreds of millions of dollars to make the Qatari aircraft, known as a "palace in the sky," suitable for use as transportation for the U.S. commander in chief, despite backers of the plan saying it would save money.

Senator Roger Wicker of Mississippi, chairman of the Senate Armed Services Committee, said the aircraft would have to be checked for surveillance equipment and receive security upgrades.

"It would be like the United States moving into the Qatari embassy," Wicker told Politico. "I’m not sure how quickly the Qatari aircraft can be retooled."

The Defense Department is already procuring a replacement for the current, aging Air Force One, with delivery by Boeing BA.N expected within a couple of years.

DEMOCRATS RESPOND

Senate Democratic Leader Chuck Schumer vowed to block all of Trump's nominees to the Justice Department until the agency reports what it knows about the Qatari offer. Three nominees are currently before the Senate.

"The attorney general must testify before both the House and Senate to explain why gifting Donald Trump a private jet does not violate the emoluments clause (of the U.S. Constitution), which requires congressional approval," he said in a speech.

The Democratic National Committee flew a plane dragging a "Qatar-a-Lago" banner near Trump's Florida Mar-a-Lago home on Wednesday.

The action angered nearby Trump supporters. "Whoever's flying that plane can kiss my ass," said Steven Drew, a visitor from North Carolina. "As far as I know, Trump's been trying to do everything to make the country as great as he can, and I'm fully on board with that," he said.

Schumer, of New York, said he wants answers to whether the Qatari government would pay for modifications needed to protect the president and secure communications.

If the U.S. government must bear those costs, Schumer said, "why are American taxpayers being asked to spend hundreds of millions of dollars or more on a plane that will only be used for year or two?"

A White House spokesperson on Monday said details of the gift were still being arranged. A new commercial 747-8 costs approximately $400 million.

Ethics experts have listed a range of Trump activities that could point to the president using his office to enrich himself or his family. Schumer mentioned a $TRUMP meme coin, plans for a new Trump hotel in Dubai and a new golf course in Qatar.

Republican Senator Rand Paul of Kentucky said Trump taking the plane would be a mistake.

“I think it's not worth the appearance of impropriety,” he told Fox News' "Jesse Watters Primetime" program. "I wouldn't take it."

(Reporting by Richard Cowan and Patricia Zengerle in Washington; Additional reporting by David Morgan in Washington and Maria Alejandra Cardona in Palm Beach; Editing by Bill Berkrot, Matthew Lewis, Alistair Bell and Deepa Babington)

((Richard.Cowan@thomsonreuters.com;))