Former New Hampshire Gov. John H. Sununu will endorse Mitt Romney, ABC News has learned.

Sununu, who also served as the White House Chief of Staff under President George H.W. Bush, will be on hand when Romney turns in his candidacy papers on Monday in Concord, N.H. Romney is also scheduled to hold a rally on site.

The campaign says Sununu will serve as chairman of the campaign’s national steering committee.

In an exclusive interview with New Hampshire’s Union Leader, which was first to break the news of the endorsement, Gov. Sununu said that he had been torn between Texas Gov. Rick Perry and Romney.

Sununu told the paper that it was Romney’s foreign policy speech in Charleston, South Carolina, that solidified his decision.

“That showed me that he understands that the principal role of the President of the United States is the security of the country and participating in trying to stabilize the world,” Sununu said.

“I am deeply appreciative of Governor Sununu’s friendship and support,” said Romney in a statement. “Governor Sununu is one of the most respected public servants in New Hampshire and he has stood for the values that are important to Granite Staters: hard work, individualism, and fiscal responsibility. His advice and counsel will be important as I work to earn support from voters in the Granite State and work to fix our troubled economy.”