By Alexandra Alper, Echo Wang and Karen Freifeld

WASHINGTON (Reuters) - A powerful U.S. national security panel reviewing Nippon Steel's $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 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 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.

(Reporting by Alexandra Alper, Echo Wang and Karen Freifeld; Additional Reporting by Jeff Mason, David Shepardson and John Geddie; Editing by Cynthia Osterman)