By Jeff Mason, Alexandra Alper and Katya Golubkova

WASHINGTON/TOKYO (Reuters) -The U.S. national security panel reviewing Nippon Steel's $14.9 billion bid for U.S. Steel let the companies refile their application for approval of the deal, a person familiar with the matter said, delaying a decision on the politically sensitive merger until after the Nov. 5 presidential election.

The move offers a ray of hope for the companies, whose proposed tie-up appeared set to be blocked when the Committee on Foreign Investment in the United States (CFIUS) alleged on Aug. 31 the transaction posed a risk to national security by threatening the steel supply chain for critical U.S. industries.

CFIUS needs more time to understand the deal's impact on national security and engage with the parties, the person said on Tuesday. Refiling sets a new 90-day clock to review the proposed tie-up and make a decision.

The review was expected to take close to the full 90 days, another person familiar with the matter said.

Japanese public broadcaster NHK reported on Wednesday that Nippon Steel had refiled an application with the CFIUS for its U.S. Steel acquisition plan by Wednesday, quoting people familiar with the matter.

Nippon Steel declined to comment. CFIUS and U.S. Steel did not immediately respond to requests for comment from Reuters.

"Extending the timeline takes some pressure off the parties and, importantly, pushes the decision past the election in November," said Nick Klein, a CFIUS lawyer with DLA Piper.

The deal has become a political hot potato. This month, Vice President Kamala Harris, the Democratic presidential nominee, said at a rally in Pennsylvania, the swing state where U.S. Steel is headquartered, that she wanted U.S. Steel to remain "American owned and operated," echoing a view held by President Joe Biden.

The White House reiterated that position on Tuesday.

Harris' Republican rival Donald Trump has pledged to block the deal if elected. Both candidates have sought to woo union votes.

Postponing the decision to after the U.S. elections will "dial down" the political temperature but does not guarantee approval, said David Boling, a former U.S. trade official who is now an analyst at Eurasia Group.

"Regardless of the CFIUS review, Nippon Steel still must reach an agreement with the United Steelworkers," Boling said. "Without that, it's very hard to see this deal happening."

The United Steelworkers Union, which vehemently opposes the deal, said on Tuesday "nothing has changed regarding the risks that Nippon's acquisition would pose to national security or the critical supply chain concerns that have already been identified."

The deal is being closely watched in Japan, a close U.S. ally and its biggest foreign investor.

"Further strengthening economic relations, including expanding mutual investment between Japan and the U.S ... are essential for both countries," Deputy Chief Cabinet Secretary Hiroshi Moriya told reporters on Wednesday.

Nippon Steel shares were 1.1% up in afternoon trade in Tokyo. U.S. Steel shares closed 0.4% down on Tuesday.

STEEL SUPPLY CONCERNS

CFIUS is concerned Nippon Steel's merger could hurt the supply of steel needed for critical transportation, construction and agriculture projects, it said in its August letter to the companies, exclusively obtained by Reuters.

It also cited a global glut of cheap Chinese steel, and said that under a Japanese company, U.S. Steel would be less likely to seek tariffs on foreign steel importers. Nippon's decisions could also "lead to a reduction in domestic steel production capacity," it said.

In a 100-page response letter to CFIUS, also exclusively obtained by Reuters, Nippon Steel said it will invest billions of dollars in U.S. Steel facilities that otherwise would have been idled, "indisputably" allowing it to "maintain and potentially increase domestic steelmaking capacity in the United States."

The company also reaffirmed a promise not to transfer any U.S. Steel production capacity or jobs outside the U.S. and would not interfere in any of U.S. Steel's decisions on trade matters, including decisions to pursue trade measures under U.S. law against unfair trade practices.

The deal, Nippon added, would "create a stronger global competitor to China grounded in the close relationship between the United States and Japan."

Robust CFIUS reviews take 90 days but it is common for companies to withdraw their filings and resubmit them to give them more time to address the panel's concerns.

According to CFIUS's 2023 annual report, 18% of companies seeking deal approval refiled their applications last year. 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, Reuters reported on Friday.

In December, CFIUS could approve the deal, possibly with measures to address national security concerns, recommend that the president block it, or extend the time table again.

(Reporting by Jeff Mason and Alexandra Alper in Washington; Katya Golubkova, Kaori Kaneko and Yuka Obayashi in Tokyo; Editing by Chris Sanders, Sonali Paul, Miral Fahmy and Tomasz Janowski)