Republican presidential candidate Donald Trump speaks during a campaign pep rally in Mobile, Ala. (Brynn Anderson/AP)

The Virginia Republican Party is considering requiring a loyalty oath from presidential primary contenders — a move widely considered an early sign of GOP skittishness about Donald Trump’s campaign.

State party officials are debating whether to require candidates to pledge their support to the eventual nominee and promise not to run as a third-party candidate — as Trump has hinted he might do.

The development could be an early sign of trouble for Trump, particularly if other state parties consider similar ideas. But it also is being debated cautiously by Republicans who worry that it could backfire and breed resentment among activists who are suspicious of attempts by the GOP establishment to control the party.

Most of all, the idea — which began to circulate the morning after the first GOP debate, when Trump indicated that he wouldn’t rule out an independent run — signals growing concern about the businessman, who has electrified the Republican field with his flamboyant and controversial candidacy.

Particularly in the key swing state of Virginia, which most Republicans think they must win to take the White House, Trump’s potential as a third-party spoiler should he not win the nomination makes some leaders nervous.

John Whitbeck, chairman of the Virginia GOP, said the loyalty oath is not directed at Trump specifically but is intended to motivate activists.

“You can’t win the White House without Virginia; you can’t win Virginia without all the Republicans in Virginia; and you can’t get the Republicans to turn out unless the hundreds of thousands of volunteers next year work their tails off,” he said. “What they’re asking before they commit to you as a candidate: Commit to them. Be part of the team.”

Corey Lewandowski, Trump’s campaign manager, declined to say whether his candidate would consider taking such a pledge.

“I don’t know anything about pledges,” he said. “A lot of people have ideas about pledges and most of them don’t get followed through, like a pledge of ‘no new taxes, read my lips’ ” — a shot at candidate Jeb Bush’s father, George H.W. Bush, who famously broke his campaign promise not to raise taxes when he was president. “Or people see it as a typical Washington, D.C., idea, to pledge to do something only to change our mind later.”

Trump’s position on a potential independent bid offended some Virginia Republicans who prize loyalty, numerous party activists said. But several said they are worried about a backlash.

“Donald Trump is like this menacing alien presence approaching Earth. Every time you shoot at him, he just keeps getting bigger and stronger — it doesn’t affect him,” said Steve Albertson, a member of the Virginia GOP’s governing board. “There’s some risk if we do something like this, some of the folks aiming directly at Trump may regret it.”

According to Politico, which first reported the loyalty oath story, Ken Cuccinelli II, a former Virginia attorney general who ran unsuccessfully for governor in 2013, has been shopping the idea to party activists. Cuccinelli said in an e-mail to The Washington Post that he supports the idea but is not pushing it, adding, “This is not a project of mine.”

“Donald Trump is an important candidate in the race for the White House and I don’t think he or anyone else should be blocked from running as a Republican,” Cuccinelli said in the e-mail. “But once the primary is over, that’s it. I support the Virginia sore-loser law that prevents candidates that lose a primary from running third-party.”

Republicans have recently sought to expand rather than restrict access to the presidential ballot. After several candidates did not make the Virginia ballot in 2012, the state legislature cut in half the number of signatures they must collect to do so, said Mike Thomas, vice chairman of the state GOP.

Politico reported that North Carolina is considering a similar loyalty oath rule. Party officials did not return a request for comment from The Post.

North Carolina, in particular, may present a challenge for Trump. The political network of conservative industrialist brothers Charles and David Koch has a strong influence in the state party as well as an aversion to Trump. Trump has not participated in most Koch-related events this year and, unlike other candidates, his campaign has not been offered valuable data about voters by Koch affiliates, said sources familiar with both operations.

The concept of a loyalty pledge among candidates — as well as voters — is not new in GOP politics nationally.

Matt Moore, chairman of the South Carolina GOP, said that for years, all candidates have signed the same basic pledge when they run for public office.

“Every South Carolina Republican candidate in the past 30 years has signed a similar statement,” he said. “No one else gets special treatment. None of the campaigns have complained to date.”

Less clear is how far party leaders are willing to go to stop Trump from participating in the process, or whether the national GOP is willing to step in at some point to ensure that he is allowed to move forward.

Republican National Committee Chairman Reince Priebus recently told a Wisconsin television station that Trump is a “net positive” who “brings a lot of interest in the Republican field.”

In Virginia, rules already may prevent an independent bid. The state Department of Elections requires all candidates to sign a form promising that their “name will appear on the presidential general election ballot only if I am nominated by the political party in whose presidential primary I am participating.”

They could still run as write-ins, department spokeswoman Rose Mansfield said.

In addition, state election law says that if a candidate running for any level of elected office is defeated in the primary, his or her name will not be printed on the ballots for that office’s general election.

Some are skeptical that the rules could survive a legal challenge.

Notwithstanding existing law, Whitbeck said he put discussion of a pledge on the agenda for next month’s meeting in the interest of transparency — and not to incite Trump.

“We want Donald Trump on the ballot. We want the excitement he brings,” he said.

But one Republican insider, who spoke on the condition of anonymity to discuss a sensitive topic, said the party is highlighting the rule to box in Trump, whose swipes against illegal immigrants have been an “unmitigated disaster” for a party trying to expand its outreach to minority voters.

“We’re saying, ‘We’re not going to let Trump hijack the party,’ ” he said.

The party’s next meeting is Sept. 19 in Richmond.

Trump, meanwhile, isn’t waiting to hit the ground in Virginia. His campaign is planning to announce new hires in the state “very shortly,” Lewandowski said.

A snapshot of Trump’s ascent in Virginia came Monday in Alexandria. Twenty-nine Trump backers from the area gathered in an office suite for a focus group conducted by Frank Luntz, a longtime Republican pollster. The intensity of support for Trump stunned Luntz as he emerged from the two-hour session. “My legs are shaking,” he told reporters. “He’s not going away. . . . This isn’t [Ross] Perot. This is much stronger than Perot.”

Participants, who did not share their last names, repeatedly told Luntz that they consider Trump a fighter who is not controlled by either political party and would be more skilled than his rivals at making deals with Congress and foreign leaders. They fumed about federal spending, especially in a time of economic unrest. And they shrugged off his past liberal positions on social issues.

Laura Vozzella contributed to this report.