Florida Sen. Marco Rubio challenged Republican front-runner Donald Trump Thursday on his stance on immigration while defending his own record in the Senate, saying he would bring about responsible immigration reform through the legal channels. Trump appeared to flip-flop on the issue, softening on his stance on H1-B visas for highly skilled workers, saying, “I’m changing. I’m changing. We need highly skilled people in this country.”

Trump has made a name for himself throughout the campaign with his strong stance on immigration, saying he will build a wall on the border with Mexico and deport undocumented immigrants. His immigration message has resonated with voters and served as a centerpiece of his campaign.

Rubio, who struggled to escape a barrage of insults from Trump throughout the debate, defended his own past support for comprehensive reform and vowed to pursue similar policies in the future. “It will be done first and foremost by bringing illegal immigration under control,” he said. “It’s not going to be an executive order and we’re not going to ram it down their throats.”

Rubio has increasingly moved away from his formerly aspirational campaign to a more aggressive approach, frequently turning to personal insults against GOP front-runner Donald Trump. “Donald is not going to make America great, he’s going to make America orange,” Rubio said at a Sunday rally, hinting that the real estate mogul used spray tans.

The Florida senator has been polling at 17.4 percent within likely Republican voters, according to RealClearPolitics, which takes the average of recently available polling data, trailing Trump by nearly 20 points and behind Texas Sen. Ted Cruz who held 19.8 percent support. Rubio came in third Super Tuesday, winning just one state, and coming in behind both Trump and Cruz.

Former Massachusetts Gov. Mitt Romney criticized the meteoric rise of Trump in a speech Thursday, urging conservative voters to look to Rubio and Ohio Gov. John Kasich, who are seen as more traditional establishment picks.

“If the other candidates can find some common ground, I believe we can nominate a person who can win the general election and who will represent the values and policies of conservatism,” Romney said, adding, “Given the current delegate selection process, that means that I’d vote for Marco Rubio in Florida and for John Kasich in Ohio and for Ted Cruz or whichever one of the other two contenders has the best chance of beating Mr. Trump in a given state.”