Progressive anger with President Biden for referring to the alleged killer of Georgia nursing student Laken Riley as "an illegal" is threatening to overshadow what was otherwise a widely celebrated State of the Union speech.

Why it matters: Biden told MSNBC he regrets using the phrase — triggering furious attacks from Republicans who say it illustrates how tone-deaf Democrats are when it comes to immigration.

• The unscripted moment from Biden's address came in response to heckling from Rep. Marjorie Taylor Greene (R-Ga.), who had earlier handed him a Laken Riley pin that read, "Say Her Name."
• "An undocumented person. And I shouldn't have used 'illegal.' It's 'undocumented,'" Biden told MSNBC when asked if he regrets using the term.

Zoom in: Former President Trump met with Riley's parents at his rally in Rome, Georgia, on Saturday, as he and other Republicans seek to use the 22-year-old student's death as a symbol of Biden's border policies.

• "Joe Biden went on television and apologized for calling Laken's murderer an illegal," Trump said to a chorus of boos at his rally. "Biden should be apologizing for apologizing to this killer."
• "To all Americans: I will seal the border. … I will stop the killing, I will stop the bloodshed, I will end the agony of our people, the plunder of our cities, the sacking of our towns," the former president vowed.

Meanwhile: Sen. Katie Britt (R-Ala.), who delivered the Republican response to Biden's State of the Union, is under major scrutiny for using the testimony of a sex trafficking victim to attack Biden's border policies.

• In reality, the victim — Karla Jacinto Romero — was forced to work in Mexican brothels during the Bush administration and was not trafficked to the U.S.
• Britt defended the anecdote on "Fox News Sunday," saying she never implied the incident took place during the Biden administration but that Biden's border policies have exacerbated human trafficking.

Go deeper: 5 key takeaways from Biden's State of the Union speech