In the wake of conservative Rep. Liz Cheney of Wyoming's ouster from her House GOP leadership position, roughly 150 Republican or former Republican leaders who are opposed to former President Trump's continued strong grip over the GOP have signed a letter threatening to leave the party if it does not reform and return to its "founding American principles."

The group's statement of principles – titled a "Call for American Renewal" – was unveiled Thursday. It outlines 13 principles to guide a "rebirth of the American cause."

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"We, therefore, declare our intent to catalyze an American renewal, and to either re-imagine a party dedicated to our founding ideals or else hasten the creation of such an alternative," the statement pledges.

The letter was spearheaded by former GOP intelligence policy strategist and 2016 independent candidate for president Evan McMullin and Miles Taylor, who served as chief of staff at the Department of Homeland Security during the Trump administration.

"I'm proud to stand with over 150 prominent Republicans and independents calling for the reform of the GOP and the creation of a new home for Americans left unrepresented by political extremism," McMullin tweeted Thursday morning.

In an interview with Fox News, he emphasized that "in the light of Liz Cheney's sacking from House leadership and the rise in extremism in the Republican Party, we collectively believe it's important to articulate an alternative principals-based vision for leadership of the party and of the country."

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McMullin said his group's manifesto "contains a vision for our desired leadership of the party and the country and what we intend to use to unite, organize and mobilize disaffected voters who don't feel well represented by the two major parties."

He argued that in the wake of the deadly Jan. 6 attack on the U.S. Capitol by right- wing extremists aiming to disrupt congressional certification of now-President Biden’s Electoral College victory over Trump, there are plenty of Republicans "who look at the Jan. 6 insurrection and say, 'That's not what I'm a part of,' who believe in fundamental American principles and who would like to get back to governing that embraces individual freedom as well as justice, that embraces truth and decency, and that is committed to preserving the American democratic republic."

The former president, in a statement, slammed the new push by what he termed "a group of RINOs and Losers who are coming out to protest President Trump."

Trump remains incredibly popular with Republican Party base voters, according to the most recent surveys. And he continues to remain incredibly influential over GOP politicians, as the former president aims to play a kingmaker’s role in internal party politics in the 2022 election cycle and as he repeatedly flirts with a 2024 run to try and return to the White House.

But McMullin claimed that "a fourth to a third of the party has desired a new direction for it. Obviously that’s still a minority of the party but it’s a significant number."

He pledged "to mobilize those people and other Americans who are aligned with these principals to have an electoral impact in 2022 and beyond. Those people are poorly represented."

And he vowed "to create a new political identity and home for them based on these principals with the intention of empowering them to decide competitive elections around the country and therefore the direction of the country as a whole."

Among the signers of the letter are four former GOP governors, including Bill Weld of Massachusetts, who unsuccessfully attempted to primary challenge then-President Trump during the 2020 election, and Mark Sanford of South Carolina, who mulled a primary challenge against Trump.

Also signing the letter are former GOP Sen. David Durenberger of Minnesota and 27 former Republican members of the House, including Barbara Comstock of Virginia, Charlie Dent of Pennsylvania and Paul Mitchell of Michigan.

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Other prominent names include attorney George Conway, a well-known anti-Trump Republican and husband of Kellyanne Conway, who managed Trump's 2016 general election campaign and served as a senior adviser in the White House; former White House communications director Anthony Scaramucci; former Department of Homeland Security official Elizabeth Neumann; former DHS general counsel John Mitnick and former New Hampshire GOP chair Jennifer Horn.

Many of those signing the letter have for months discussed forming a splinter Republican Party in the wake of the storming of the Capitol.

McMullin, Taylor and over 100 other people held a virtual meeting in February to explore forming a new political party or a new bloc within the GOP. Rep. Adam Kinzinger of Illinois, who along with Cheney was one of the 10 House Republicans to vote in January to impeach Trump for his role in inciting the insurrection, also attended the meeting. A straw poll taken during that gathering suggested there was a divide between launching a new and separate political party or forming a faction within the GOP.