WASHINGTON, Sept 13 (Reuters) - A powerful U.S. national

security panel reviewing Nippon Steel's ( NISTF ) $14.9 billion

bid for U.S. Steel faces a Sept. 23 deadline to recommend

whether the White House should block the deal, two people

familiar with the matter said.

The Committee on Foreign Investment in the United States has

until that day to complete its second 90-day review of the

Japanese company's proposed takeover of U.S. Steel, the people

said, declining to be named because the matter was not public.

On that date, CFIUS officials could grant a request by the

companies to extend the review another 90 days, which would

defer the politically sensitive decision until after the Nov. 5

election.

If the panel does not do that, it could approve the deal,

possibly with measures to address national security concerns, or

it could recommend President Joe Biden block it.

The deal has become a political hot potato and faces

high-profile opponents including Biden, Democratic presidential

candidate and Vice President Kamala Harris and Republican Donald

Trump. They oppose foreign ownership of U.S. Steel, which

produces a strategically and symbolically important commodity

used to build ships, trains and infrastructure.

"The president's position is that it is vital for U.S. Steel

to remain an American steel company that is domestically owned

and operated," White House adviser Saloni Sharma said.

U.S. Steel is headquartered in Pennsylvania, a crucial swing

state in the presidential election. The United Steel Workers

union, which endorsed Harris, opposes the deal.

As the clock ticks down to the Sept. 23 deadline, the

politics and uncertainty surrounding the deal are in the

spotlight. The companies have sought to save the acquisition

after the panel said it would harm the security of U.S. steel

supplies in an Aug. 31 letter seen by Reuters.

The companies countered in a 100-page letter, also

exclusively reported by Reuters, the deal would increase U.S.

steel output and asked for an extension to address concerns.

CFIUS and Nippon Steel ( NISTF ) declined to comment and U.S. Steel

did not respond to a request for comment.

A decision is not expected in the coming days, according to a

senior administration official. The Washington Post reported on

Friday a decision could be postponed until after the election.

Sources said the companies hope recent support for the deal,

including a letter from business groups like the Chamber of

Commerce raising concerns the transaction is being influenced by

political pressure, could turn the tide.

Robust CFIUS reviews take 90 days but it is common for

companies to withdraw their filings and resubmit them to give

the companies more time to address the panel's concerns. This

resets the 90-day clock.

Nippon Steel ( NISTF ) and U.S. Steel filed for the review in March,

and CFIUS allowed them refile in June, starting a second 90-day

clock that runs out on Sept. 23, sources said.