President Trump’s embattled national security adviser Michael Flynn resigned Monday night and three names have emerged as possible replacements.

Vice Adm. Bob Harward is one name that has come up to replace Flynn as national security adviser, and the leading candidate to get the job, a senior official told Fox News.

Harward is a U.S. Navy SEAL, but also has a previous relationship with Secretary of Defense James Mattis. Harward was the deputy commander of the U.S. Central Command under Mattis and was also the deputy commander of U.S. Joint Forces Command.

He also served on the National Security Council for President George W. Bush and commissioned the National Counter Terrorism Center.

TRUMP’S NATIONAL SECURITY ADVISER MICHAEL FLYNN RESIGNS

White House sources described Harward as the “toughest guy in the SEALs” and a “real rock.”

A senior administration official added that if Howard is the choice to replace Flynn, he could be in place by the end of the week.

Lt. Gen. Joseph Keith Kellogg, Jr. has been floated as a permanent replacement for Flynn. Trump named him the acting national security adviser after Flynn resigned.

Kellogg is a decorated U.S. Army veteran, having served from 1967 to 2003. He earned the Silver Star, the Bronze Star with “V” device and the Air Medal with “V” device during his time in the Vietnam War.

Kellogg was chief operating officer of the Coalition Provisional Authority in Iraq, the interim governing body following the fall of Saddam Hussein in 2003. He previously worked as executive vice president of research and technology for Virginia-based information technology firm CACI International, which works as a contractor for defense, intelligence and homeland security agencies.

Another name floated as a possible replacement for Flynn is retired Gen. David Petraeus.

Trump routinely dropped Petraeus’ name during his election campaign. Trump said that Petraeus was punished more severely for leaking classified documents to his mistress than Hillary Clinton was punished for setting up a private email server during his time as Secretary of State.

READ: MICHAEL FLYNN’S LETTER OF RESIGNATION AS NATIONAL SECURITY ADVISER

Petraeus was briefly considered for the secretary of state job, but was passed up because of his rocky tenure as CIA chief and the possibility that he wouldn’t be confirmed in the Senate because of those issues.

An administration source told Fox News that Petraeus would be a "bad choice" for the NSA job considering he was still on "federal probation" and would give Trump critics ammunition to cry hypocrisy, given Trump's frequent lambasting of Clinton's classification issues.

According to the Washington Examiner, Bush’s former national security adviser Stephen Hadley and Tom Bossert, a former national security aide under Bush have been considered a Flynn’s replacement. The paper added that Adm. James Stavridis, a dean at Tufts University, is also on the table.

Flynn’s resignation ended speculation about his fate following reports he had misled Vice President Pence and other officials about his contacts with Russia.

Flynn conceded that discussions of sanctions may have come up during several calls with the Russian ambassador during the transition period leading up to Trump's Jan. 20 inauguration.

He acknowledged that he gave "incomplete information" about those discussions to Pence who, apparently relying on information from the national security adviser, initially said Flynn had not discussed sanctions with the Russian envoy.

Whoever emerges as Trump's choice will take the helm of the National Security Council at a time when the young administration is grappling with a series of national security challenges, including North Korea's reported ballistic missile launch. The president, who was joined at his Mar-a-Lago estate by Japanese Prime Minister Shinzo Abe over the weekend, voiced solidarity with Japan.

The White House is also dealing with fallout from the rocky rollout of Trump's immigration executive order, which has been blocked by the courts. The order was intended to suspend the nation's refugee program and bar citizens of seven Muslim-majority countries from entering the United States.

Fox News’ John Roberts and Lucas Tomlinson and the Associated Press contributed to this report.