While Sen. Lindsey Graham (R-S.C.) is not regarded as a top-tier presidential candidate but he could be a potentially disruptive force in a fluid race. (Susan Walsh/AP)

The already-crowded field of would-be Republican presidential candidates grew again on Thursday when Sen. Lindsey O. Graham (R-S.C.) announced the formation of an exploratory committee for president.

The launch of the Security Through Strength committee enables Graham to raise money for a potential run. It’s the clearest sign yet that he is serious about entering the race, and comes as a flurry of White House aspirants are taking public and private steps toward entering what could be the most wide-open GOP primary in memory.

Graham is not regarded as a top-tier candidate by most Republicans and may run into problems because of some moderate views he holds.

But he is known primarily as one of the GOP’s leading hawks on national security and represents an early nominating state, making him a potentially disruptive force in a fluid race. His positions on the use of U.S. force will probably put him at odds with at least one other prominent 2016 hopeful, Sen. Rand Paul (R-Ky.), who represents the growing libertarian wing of the party.

Addressing reporters at the Capitol, Graham said he was optimistic that his brand of Republican politics could be a good fit for the country.

“I am confident that conservatism can win a national election, that my form of conservatism has been accepted in South Carolina, and maybe it will be accepted outside of South Carolina,” he said. “I won’t know until I try.”

Sen. Ted Cruz of Texas, another Republican making moves toward running, called Graham a “good man,” a “friend” and a “man of deep passions.”

Cruz and Paul are laying the groundwork for a possible run, as are former Florida governor Jeb Bush, New Jersey Gov. Chris Christie and Sen. Marco Rubio of Florida, to name a few other GOP hopefuls.

“I think it is entirely likely that we will see a crowded field,” Cruz said. “And that’s a healthy thing.”

Whether the field has room for a candidate running primarily on national security is what Graham will be testing in the coming months.

“That concept of securing America economically and militarily is sort of the theme of the campaign,” he said.

The South Carolinian’s biggest cheerleader is his close friend, Sen. John McCain (R-Ariz.), who lost to Barack Obama in 2008 as the GOP presidential nominee. McCain enthusiastically pledged to help Graham.

“He’s a dark horse,” McCain said. “Keep an eye on him. In debates, he’ll shred ’em.”

Graham is one of President Obama’s most vocal foreign policy critics, arguing for more forceful military action in Syria and Iraq and often making his case on Sunday morning news shows.

“I don’t think you can have peaceful coexistence with radical Islam,” Graham said. “But you can have security. And security is economic prosperity, for not a few, but for everybody.”

One potential vulnerability for Graham is his centrist stance on immigration. He was part of a bipartisan group of senators that pushed a comprehensive reform measure in the last Congress. The bill passed the Senate but failed in the House, where it was torpedoed by conservatives who opposed a path to citizenship for undocumented immigrants.

Sen. Charles E. Grassley (R-Iowa) estimated that immigration could be a problem for Graham among “15 percent” of Republican caucusgoers in his state, which will host the first-in-the-nation caucuses in 2016.

Another possible trouble spot: Graham’s votes to confirm Supreme Court Justices Sonia Sotomayor and Elena Kagan.

But he didn’t sound worried.

“I won by 41 points against six opponents,” Graham said, referring to his 2014 primary. “I am conservative by any rational definition. Working with the other side when it makes sense is not inconsistent with being conservative.”

South Carolina’s other Republican senator, Tim Scott, said he learned of Graham’s plans “several weeks ago.” But Scott — whose endorsement will be widely sought because of the state’s early primary — isn’t siding with anyone yet.

“[The field] is probably going to get smaller after it gets larger,” he said. “And I would imagine that Lindsey will be a part of the conversation for several months to come.”

Democrats said Graham’s move is a sign of a weak Republican field.

“Jeb Bush and Mitt Romney took early steps to try to scare people out of the race. Instead, they are scaring people in,” Democratic National Committee spokesman Mo Elleithee said in a statement.

A packed field is fine by Grassley, whose state will host many candidates in the coming months.

“The more the merrier. The better it is for the economy of Iowa,” he said.

Paul Kane contributed to this report.