President Trump went all-out for South Carolina Gov. Henry McMaster Monday night, taking a political risk to stump for the vulnerable Republican the day before a heated primary runoff.

Speaking at Airport High School in West Columbia, S.C., on a rainy, sweltering evening, Trump cheered his summit with North Korean leader Kim Jong Un and called again for strong border security -- but also gave a full-throated endorsement of McMaster, whom he called a "handsome guy" and a "fighter."

"Get your asses out tomorrow and vote," Trump told the rally. "He's a great man."

Protesters and counter-protesters assembled outside several hours before the speech. Tensions were high, as President Trump earlier in the day accused Democratic Rep. Maxine Waters, D-Calif., of openly advocating “harm” against his supporters.

On Monday night, Trump briefly referenced Waters' threatening language, saying the Democratic Party has become the "party of Maxine Waters" as boos filled the gymnasium.

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McMaster took a big leap when he endorsed Trump for president in January 2016, long before the billionaire businessman had defeated his 16 more conventional primary foes.

The South Carolina governor was with Trump "right from day one," the president said at Monday's rally.

Trump also addressed the ongoing debate about border enforcement and illegal immigration on Monday, blasting Democrats for supporting "open borders" and "illegals coming into the country -- some of whom are not good."

"It's not 'build that wall' anymore," the president said, after the audience began chanting. "It's continue building that wall."

Trump also touted "Twin Peaks" creator David Lynch's statement over the weekend that Trump "could be one of the greatest presidents in history," before noting that the filmmaker's "career in Hollywood is officially over" for daring to support the president publicly.

The president then derided "Tonight Show" host Jimmy Fallon after the comic apologized for "humanizing" the president during the presidential campaign. Trump told the late-night host to "just be a man" and insisting that he had brought Fallon's show massive ratings.

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As in so many Republican primaries this year, the gubernatorial race in South Carolina may indeed hinge on which candidate can most convincingly align with Trump – though both McMaster and rival John Warren, a businessman, are running on similar platforms.

Earlier this month, Trump-bashing Rep. Mark Sanford, R-S.C., was ousted in a primary that focused largely on his constant, harsh criticisms of the president. Hours before polls closed in that race, Trump derided Sanford on Twitter and endorsed his rival, state Rep. Katie Arrington.

On Monday night, Trump called Arrington, who was critically injured in a car accident over the weekend, a "very special" person and said "she's going to be back, very soon."

Trump also did a victory lap over Sanford's loss, calling the ousted congressman a "guy I don't like too much."

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"Get your asses out tomorrow and vote." — President Trump in South Carolina

The White House has been putting on a strong show of support for McMaster. Trump's Monday rally comes on the heels of a similar visit Saturday by Vice President Mike Pence.

McMaster received the most votes in a June 12 primary but fell short of the 50 percent needed to win the nomination outright, giving Warren an opening.

Trump, predicting a "red wave" in the fall mid-term elections, has been holding rallies and campaign events on behalf of several Republicans in recent weeks. The rallies have touched on common themes, including North Korea, health care, the new military Space Force, and trade.

At one point Monday, Trump suggested that ailing Arizona Sen. John McCain may have been "grandstanding" with his dramatic "no" vote on a key Obamacare repeal vote last year.

Just weeks after his election, Trump selected then-S.C. Gov. Nikki Haley as his ambassador to the United Nations. Her departure cleared the way for McMaster in early 2017 to ascend to the governorship he had sought since losing a bruising primary to none other than Haley seven years earlier. Now, he seeks a full term on his own merit.

The winner of Tuesday's runoff in South Carolina will face Democratic state Rep. James Smith in fall election. The Republican-leaning state hasn't elected a Democratic governor since Jim Hodges in 1998.

Fox News' Terace Garnier and The Associated Press contributed to this report.