A campaign to stop Donald Trump from becoming the Republican presidential nominee has the support of nearly 400 delegates to the GOP’s convention next month, according to organizers, quickly transforming what began as an idea tossed around on social media into a force that could derail a national campaign.

While organizers concede their plan could worsen internal party strife, they believe they are responding to deep-rooted concerns among conservatives about Trump, who is suffering from declining poll numbers after weeks of missteps and embarrassing headlines.

“Short-term, yes, there’s going to be chaos,” said Kendal Unruh, a co-founder of the group, Free the Delegates. “Long-term this saves the party and we win the election. Everything has to go through birthing pains to birth something great. We’re going to go through the trauma of the birthing pains, but the reward will be worth it.”

Unruh, of Colorado, said her cause is winning support from “the non-rabble rousers. The rule-following, churchgoing grandmas who aren’t out protesting in the streets. This is the way they push back.”

The campaign’s growing support came amid a significant shake-up in Trump’s campaign. On Monday, he fired his campaign manager, Corey Lewandowski, one of his most loyal and vocal aides. The move was seen as an urgent attempt to ease GOP concerns over the campaign’s direction.

Presumptive Republican nominee Donald Trump fired embattled campaign manager Corey Lewandowski on June 20. The Fix's Chris Cillizza breaks down what comes next. (The Washington Post)

Unruh and other GOP delegates from Colorado hatched the idea of trying to stop Trump by introducing a rule change: Instead of binding delegates to the results of the caucuses and primaries — as many party leaders insist they are — the convention’s 2,472 delegates should instead be able to vote their conscience and select whomever they want.

For weeks, Unruh, her colleague Regina Thomson and other Colorado Republicans sought out like-minded delegates in other states. After Unruh appeared in newspaper interviews and called in to a few radio talk shows, she said other delegates with similar concerns in places such as Louisiana and Missouri reached out. By this past weekend, Unruh was consulting a lawyer about possible fundraising plans while Thomson was compiling the list of interested delegates, building a website and booking a conference call phone line that could host 1,000 participants.

“As we carefully consider not only the presidential nominee but the rules of the convention, the platform of the Republican Party and the vice-presidential nominee, remember that this is true reality TV — it is not entertainment,” Thomson said on a conference call she hosted late Sunday.

Thomson said at least 1,000 people participated in the call. Delegates who participated said they plan to spend this week wooing others to the cause.

“Trump claims to be pro-life, but he used to be pro-abortion. He claims to be for traditional marriage, but he never used to talk that way. His lifestyle is such that I cannot support him,” Utah delegate Gayle Ruzicka said. “Trump doesn’t even seem to understand Christian principles.”

Talmage Pearce, a delegate from Arizona, said he is backing the movement because Trump’s “deceit, bullying, insulting, blackmailing and liberal views all make it impossible for me to cast him my endorsement.”

In recent days Trump has called attempts to strip him of the party nomination “totally illegal” and a rebuke of the millions of people who voted for him. Over the weekend he accused former opponents Jeb Bush and Sen. Ted Cruz (Tex.) of trying to undermine his candidacy. Both men say they have nothing to do with the new movement.

On the call Sunday night, leaders of Free the Delegates repeatedly insisted they are not working on behalf of any of Trump’s former opponents. They also lashed out at Chairman Reince Priebus and other officials at the Republican National Committee who have dismissed the delegates’ efforts as silly and a media-driven myth.

“Mr. Priebus needs to understand that leadership has not answered the call of the most important people in the Republican Party, and that’s the conservatives. We have always been there; we’ve endured a lot of one-way loyalty,” said Chris Ekstrom, a Dallas-based businessman and founder of Courageous Conservatives PAC, which supported Cruz’s campaign but is now backing the new movement.

Steve Lonegan, a Republican consultant from New Jersey who is advising the dissidents on fundraising and media outreach, asked participants in the call to donate to Ekstrom’s PAC. The money would be used to help track down more delegates and to help any delegates who may face threats or pressure back home.

Delegates in several states are under pressure not to join anti-Trump groups. In North Carolina, some have proposed fining delegates or kicking them out of the party if they vote against Trump. In other states, party leaders are threatening to strip delegates of their credentials if they buck primary results and vote against Trump, according to delegates who have contacted The Post. Some reached out on the condition of anonymity, saying that their spouses are fearful of physical threats if they speak out publicly.

But several delegates said they were buoyed by House Speaker Paul D. Ryan (R-Wis.), who told NBC’s “Meet the Press” on Sunday that “it is not my job to tell delegates what to do, what not to do, or to weigh in on things like that. They write the rules. They make their decisions.”

Said Unruh: “Paul Ryan signed our permission slip.”

One delegate from Colorado supporting the campaign, who requested anonymity to avoid harassment, wrote in an email that “we will not put our delegates in an ethical dilemma” if they are unbound. “We live in America. The land of the free. As delegates, we should be free to vote our conscience.”

Cecil Stinemetz, a delegate from Iowa angered by what he views as intimidation tactics, released an email he received on Friday from Steve Scheffler, who holds one of Iowa’s seats on the Republican National Committee and is a leader of the Iowa Christian Alliance.

“Stop this madness Cecil!!” Scheffler wrote. “All the other candidates have either folded their campaigns or suspended them. You are hurting Iowa! Can’t you behave yourself? You are an embarrassment! The binding for Iowa is what it is and your trying to make a name for yourself in the press is disgusting! Christians don’t behave this way!”

Scheffler declined to comment about the exchange.

“My whole adult life I have been a loyal Republican. But this whole experience has really opened my eyes to what some folks I previously thought were nuts were warning us about,” Stinemetz said. “If you want to know how it’s possible for someone like Donald Trump to rise this far in our party, it’s because we have leaders like this.”