With just hours to go before a pivotal special election in Mississippi that will determine the strength of the GOP's Senate majority next year, President Trump on Monday evening touted NASA's Mars landing, vowed to redouble his border security efforts, and accused some caravan migrants of using children as human shields.

His comments came at a fiery rally in Tupelo, Miss. for incumbent GOP Sen. Cindy Hyde-Smith, who is facing off in Tuesday's special election runoff against Democrat Mike Espy, and at a roundtable shortly afterwards. Trump later held another rally in Biloxi hours later Monday evening, underscoring the importance of the race for Senate Republicans.

In keeping with the Christmas spirit, Trump entered the second rally through a wall that appeared to look like a chimney as fake snow rained down from above.

Voting for Hyde-Smith is "so important," Trump told the cheering crowd in Tupelo, as Republicans look to expand their Senate majority to 53 seats.

In between the Tupelo and Biloxi rallies, Trump attended a roundtable with Mississippi law enforcement leaders on his bipartisan criminal justice reform effort. There, he charged that some migrants -- whom he identified as "grabbers" -- rushed the San Ysidro, Calif., point of entry on Sunday, essentially using children as human shields at the border. In a statement late Monday, Homeland Security Secretary Kirstjen Nielsen backed up Trump's comments.

TRUMP, DHS ACCUSE MIGRANTS OF USING CHILDREN AS HUMAN SHIELDS AT THE BORDER

Hyde-Smith was appointed in March to fill retired Republican Sen. Thad Cochran's seat, and is now seeking to finish out the last two years of his term. Espy, who served in former President Bill Clinton's administration, is seeking to become Mississippi's first black senator since Reconstruction.

"Your vote on Tuesday will decide whether we build on our extraordinary achievements, or whether we empower the radical Democrats to obstruct our progress," Trump said.

The president was joined on stage at one point by South Carolina Republican Sen. Lindsey Graham, who told him he has "done a hell of a job."

"We're sending a clear message to the caravans, to the trespassers -- go back home." — President Trump

"If you like [Supreme Court Justice Brett] Kavanuagh, there's more coming," Graham, one of Kavanaugh's staunchest defenders during his Senate confirmation hearings earlier in the year, told the crowd. "Let's win tomorrow." (The Senate has exclusive authority to confirm all federal judicial appointments.)

The president briefly pivoted to discuss Sunday's confrontation at the border, when hundreds of caravan migrants rushed the port of entry at San Ysidro, Calif., and were dispersed with tear gas from U.S. Border Patrol authorities.

"Are we doing OK on the border, folks?" Trump said to cheers of "Build that wall."

"We're not going to have it -- you've got to come into our country legally," Trump said. "We have a lot of [the wall] built, and it's going up. And the rest of it -- it's pretty nasty looking wire, isn't it? We're doing well."

He added: "We're not letting people into our country unless they come in legally -- and we want people to come in through merit. We have great companies coming into our country. We need workers. We want them to come in. But they have to be talented people who can love our country -- they have to come in through merit, not through a [visa] lottery."

Criticizing migrants who waved their home country's flags and threw rocks at U.S. officials -- even as they sought entry into the U.S. -- Trump unequivocally condemned Sunday's attempted incursion at San Ysidro.

"We will not tolerate any form of assault or attack upon our border agents, like happened yesterday," Trump said. "We're sending a clear message to the caravans, to the trespassers -- go back home."

He then touted historically low African-American unemployment rates and told the crowd that "we made history" on Election Day "by expanding our Senate majority."

For her part, Hyde-Smith said she was "honored" by Trump's visit and emphasized her support for the Second Amendment and the president's judicial nominees.

"I've worked very, very hard for you," Hyde-Smith told the crowd. "I will stand for your conservative values, and that is what is on the ballot tomorrow."

"We have reawakened NASA. And that's a good thing." — President Trump

WATCH: HUNDREDS OF MIGRANTS RUSH PORT OF ENTRY AT SAN YSIDRO, MET WITH TEAR GAS

After discussing his plans for a Space Force, Trump highlighted the historic landing of a NASA spacecraft on Mars earlier in the day.

"Today, we just landed on Mars, did you hear that?" Trump said. "They were celebrating at NASA. We have reawakened NASA. And that's a good thing." He added: "A lot of the rich guys like rockets. So we charge them rent. Just make sure you have an American flag on the rocket."

He then immediately turned to the Iran nuclear deal, which he called "one-sided" and pulled the U.S. out of earlier this year, and touted his decision to move the U.S. Embassy to Jerusalem.

Large crowds gathered hours in advance of the rally at Tupelo Regional Airport, where GOP Rep. Trent Kelly led supporters in a chant of "Build the wall" as Air Force One approached for landing.

The president carried Mississippi over Hillary Clinton by nearly 20 percentage points in the 2016 presidential election, and GOP Sen. Roger Wicker won re-election by a similar margin earlier in the month.

Hyde-Smith also remains popular among black Mississippi Republicans, according to local reports.

But Trump cautioned voters not to make assumptions or stay home. "Don't take any chances," he warned the crowd in Tupelo Monday evening.

And Democrats have seen some cause for optimism in recent weeks, fueled by a series of missteps by Hyde-Smith.

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The incumbent Republican lawmaker was recorded during a campaign stop saying that if a supporter invited her to a "public hanging," she would be in "the front row.” She has since said her comment was made in jest and denied any racial connotation.

Speaking to reporters outside the White House earlier Monday before flying to Mississippi, Trump said Hyde-Smith "felt very badly, and she certainly didn't mean that, and she's already apologized and I think very strongly."

He said her comments were "taken a certain way, but she certainly didn't mean it."

"I've known her for a period of time now as a senator -- she's been an excellent senator," Trump said. "I'm going there -- I'm going to make, I guess, two rallies on top of everything else. And I hope you're all coming."

Amid fallout from those remarks, Major League Baseball asked that Hyde-Smith return the organization’s $5,000 donation. Other organizations have made similar requests.

Hyde-Smith also co-sponsored a bill in the Mississippi state Senate in 2007 that would have honored a former Confederate soldier for his efforts to "defend his homeland."

The resolution, which was first reported over the weekend, called a Mississippi resident identified as Effie Lucille Nicholson Pharr "the last known living 'Real Daughter' of the Confederacy living in Mississippi" and praised her father's work to "defend his homeland."

Meanwhile, Hyde-Smith's campaign sharply pushed back against a report in the Jackson Free Press over the weekend that she had attended what was described as a "segregation academy" in the 1970s to avoid studying with black students, calling it a "new low."