At his final rally of the 2018 midterm season, Donald Trump stuck with familiar topics, railing against migrants at the southern US border as potential violent criminals and gang members, and accusing Democrats of intentionally letting them into the US to take jobs and ruin healthcare for US citizens.

The president told a rapt audience of supporters in Biloxi, Mississippi, on Monday night that his message for migrants was simple: “Turn back now, go back home, we will not let you in,” prompting the crowd to erupt into a cheer of “build that wall”.

It was the second of two rallies at which Trump spoke in support of Republican Cindy Hyde-Smith in Tuesday’s runoff election for the Senate. At the earlier event in the north-east Mississippi city of Tupelo, Trump said: “We don’t want those people in Mississippi. I’m sorry,” while describing the migrants as “tough” people.

“Democrats have become the party of caravans and crime,” he added.

He continued in Biloxi, saying that Democrats “want to turn America into one giant sanctuary city for violent criminals and MS 13 and other gang killers”, and accused Hyde-Smith’s Senate opponent of supporting “open borders”.

“You know you hear these stories about, oh, they don’t commit crimes, we commit crimes, they don’t commit crimes. Always it’s us. No, no, it doesn’t work that way. It’s fake news,” Trump said, contrary to research that concludes immigrants commit less crime per capita than native-born citizens.

At a roundtable discussion he attended between the two rallies, Trump also defended the apparent use of teargas on migrants at the southern border on Sunday, asking why mothers had put themselves and their children in harm’s way.

“You really say, ‘Why is a parent running up into an area where they know the teargas is forming and it’s going to be formed and they were running up with a child?’”

Trump’s effort to capitalize on immigration fears marked a return to the strategy he employed in the run-up to the November midterm elections, and it played with well rally-goers.

“He’s just telling it like it is, he’s honest that these people ain’t all good like the media tries to tell us,” said Carson Graham while holding a “Merry Christmas” Trump campaign sign.

It was a festive affair in Biloxi, with green and white Christmas trees and even a blast of fake snow in the arena when Trump entered. “Are you sure this is indoor, that beautiful snow looks so real,” Trump said, bemused. Before he came out, a Santa and Mrs Claus wearing “make America great again” hats tossed out holiday-themed Trump-branded apparel to the crowd.

Hyde-Smith took the stage too, and gave rallygoers a quick pitch on her conservative bona fides, bullet-pointing her anti-abortion, pro-second amendment and pro-law enforcement platform. Trump was the main event, however, and all the speakers mostly focused on how electing Hyde-Smith would mean another reliable vote for the president’s agenda in the Senate.

“She’s been there every time we needed her and [she’s] supported this president every step of the way,” said Vice-President Mike Pence before ceding the stage to Donald Trump.

Hyde-Smith was appointed to the Senate by Governor Phil Bryant when Thad Cochran retired earlier this year. The winner of Tuesday’s runoff between Hyde-Smith and Democrat Mike Espy will serve the final two years of the term.

While she leads in polls, Hyde-Smith’s campaign has been mired in controversy over the past few weeks over comments about issues of race. Most notably she has drawn fire for a video showing her praising a supporter by saying: “If he invited me to a public hanging, I’d be on the front row.”

Mississippi has a history of racially motivated lynchings.

Trump said Hyde-Smith had apologized and misspoke. He said her comments were “taken a certain way but she certainly didn’t mean it”.

Trump meanwhile attacked Espy as “far left”, telling supporters in Tupelo: “Oh, he’s out there. How does he fit in with Mississippi? … How does he fit in?”

A former US secretary of agriculture, Espy continues to emphasize that he is a moderate seeking the votes of everyone and willing to work across party lines. He noted that he had crossed the “party chasm” to endorse the re-election of the Republican governor Haley Barbour in Mississippi in 2007.

Meanwhile, a civil rights group has challenged Mississippi’s absentee voting procedures in a lawsuit filed on the eve of the Senate runoff election.

The Washington-based Lawyers’ Committee for Civil Rights Under Law is asking a federal court to make Mississippi extend its deadline for voters to return absentee ballots.

The complaint says some voters did not have enough time to fill out and mail absentee ballots for the runoff over the Thanksgiving holiday unless they paid for costly overnight shipping.

The group sued on behalf of the Mississippi NAACP and three voters. The secretary of state’s office did not immediately return an email seeking comment.

The Associated Press contributed to this report.