“You know, we’ve got a saying in the South that says it’s not the dog in the fight that’s important, it’s the fight in the dog that’s important,” McMaster told the crowd of about 1,100. “Well this dog’s got plenty of fight — and it’s gonna take some fighting.”

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The endorsement comes as the flamboyant real estate mogul recalibrates his blustering anti-establishment pitch, promising to shake up the way Washington works while also telling voters that he will work with Democratic and Republican leaders if he is elected. McMaster — a former South Carolina GOP party chairman with a long history of holding elected office in the state — is the highest-ranking elected official in the country to endorse Trump in the Republican presidential primary; his support serves to highlight the billionaire’s increasing appeal to Republican leaders in Washington.

The thawing relationship between Trump and the Republican establishment has followed the ascent of Sen. Ted Cruz (R-Tex.) in national polls, an alarming rise for Republican leaders who see Cruz as a self-serving obstructionist. With the Iowa caucuses just days away, Trump and Cruz are locked in an escalating battle for undecided voters in the Hawkeye State and beyond. Trump is now aggressively drawing a contrast with Cruz by presenting himself as an experienced dealmaker while criticizing Cruz for failing to work with his colleagues in the Senate.

Trump's unity message has been met with incredulity from critics who have accused the candidate of stoking divisions within the country, particularly regarding his rhetoric on illegal immigration and his calls to temporarily ban all Muslims from entering the country.

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But the billionaire showman has walked a fine line, making it clear to his supporters that he is a thoroughbred conservative — which Cruz has repeatedly called into question — but insisting that he is open to legislative negotiations with Democrats and establishment Republicans alike despite their differences.

In many cases, Trump's most personal attacks are lobbed at fellow Republicans.

"You know, look, we know where the Democrats are coming from, okay? We understand that. The problem we have is these Republicans that we send to Congress," Trump said.

The endorsement from McMaster, a longtime ally of Gov. Nikki Haley, caught many Palmetto State Republican strategists by surprise — in no small part because of the simmering feud between Trump and Haley. The Republican governor’s party-sanctioned response to President Obama’s State of the Union address this month made pointed jabs at the tone and tenor of Trump’s campaign.

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The Trump campaign has unveiled several endorsements in recent weeks, perhaps most notably that of former Alaska governor Sarah Palin, who joined Trump on the campaign trail in Iowa and Oklahoma.

On Tuesday, Trump was endorsed by Liberty University President Jerry Falwell Jr., son of the famed televangelist, a boon for Trump as he tries to shore up support among Christian evangelicals. He was also endorsed by conservative firebrand Joe Arpaio, the controversial sheriff of Maricopa County, Ariz., best known for his hard-line efforts to curb illegal immigration.