Highlights from South Carolina Sen. Lindsey Graham's announcement that he's seeking the Republican nomination for president in 2016. (Video: AP)

This story was updated at 11:13 a.m.

Sen. Lindsey O. Graham (R-S.C.) announced Monday in his home town of Central, S.C., that he will run for president in 2016, adding to a crowded Republican field one of the party's most aggressive national security voices in a campaign that has focused often on foreign policy.

“I am running for president of the United States because I am ready to be commander in chief on day one … to defend our nation with a sound strategy, a strong military, stable alliances and a steady determination,” Graham told a crowd of supporters. “I have more experience with our national security than any other candidate in this race. That includes you, Hillary.”

Graham's decision, widely expected in recent weeks, is the culmination of months of exploration. He launched an exploratory committee in January called "Security Through Strength," a reflection of his position as one of the most hawkish figures in the Republican Party. Last month, he signaled in a television interview that he was about to make an announcement about his future.

Graham enters the field as a staunch military hawk in a race where many of his rivals have already been emphasizing tough postures toward terrorists and foreign leaders who oppose the United States and its allies. The list of challenges he'll face is long: it begins with criticism from some conservative activists who say he has been too liberal -- and includes a long poll climb out of low single-digit territory.

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The South Carolina senator has routinely lambasted President Obama’s strategy abroad -- along with the foreign policy views of potential GOP presidential rival Sen. Rand Paul (R-Ky.). He has called for more forceful military strategies in Iraq and Syria and warned the Obama administration against trying to reach a nuclear deal with Iran.

[Lindsey Graham calls Obama a 'flawed negotiator']

[GOP's Lindsey Graham sees an opening as antagonist to Rand Paul]

Some of the senior strategists expected to play a key role in Graham's campaign once worked for Sen. John McCain (R-Ariz.), Graham’s closest friend in the Senate. McCain, the 2008 Republican presidential nominee, has vowed to do whatever he can do get Graham elected.

"Senator Graham, he's my man," McCain gleefully told reporters in January when Graham launched his exploratory committee. He later added: "I don't want to raise expectations, but I am confident in a debate he will make an impression on the American people.”

[McCain, Graham fire back at Rand Paul]

Christian Ferry, who was deputy campaign manager for McCain in 2008, is expected to manage Graham's campaign. Jon Seaton, a top political strategist for Graham, also worked for McCain.

Graham is expected to face some skepticism from conservative activists in the campaign over his support for comprehensive immigration reform, his votes for Obama's Supreme Court nominees and his criticism of the tea party movement.

Graham pushed a sweeping immigration reform bill in 2013 that included a path to citizenship for undocumented immigrants. Many hard-line conservatives oppose such a path. The bill Graham pushed alongside a bipartisan group of senators never became law as resistance on the right led the GOP-controlled House to ignore it.

The South Carolina senator’s votes to confirm Supreme Court Justices Sonia Sotomayor and Elena Kagan could also be a vulnerability for him among conservatives. And his blunt assessment of the tea party movement in its early days could compound his problems.

“The problem with the tea party, I think it’s just unsustainable because they can never come up with a coherent vision for governing the country. It will die out," Graham told the New York Times Magazine for 2010 story.

Graham is the 40th most conservative senator, according to National Journal's 2013 vote ratings.

Tea Party activists were eager to unset Graham in his 2014 primary. But they failed to rally around any single alternative. Graham won the crowded primary with little trouble and was easily reelected in November.

When he launched his exploratory committee, Graham sounded confident that his political resume would hold up on the campaign trail.

"My party is center right. They are not in the right ditch. There’s an element of the left and the right that I don't think reflects both parties. I won by 41 points against six opponents," he told reporters.

He later added: "I will never concede to anyone that conservatism requires a hands-off approach to solving problems."

Graham, 59, is a native of South Carolina, an all-important early voting state. He is a life-long bachelor and an Air Force veteran and lawyer who worked his way up from the state House to the U.S. House in the 1990s. He was first elected to the Senate in 2002.

The senator is generally chatty with reporters in the halls of Congress, and he has maintained a major presence on Sunday morning news shows in recent years.

The New York Times blog The Upshot analyzed data collected by American University and found that between 2009 and the fall of 2014, Graham made 85 appearances on Sunday shows. Only McCain appeared on the shows more often.

Jose A. DelReal contributed to this report.