Here's the one issue that could be difficult for Marco Rubio to play off during the debate
During the third Republican debate last month, Rubio managed to turn former Florida Gov. Jeb Bush's (R) attack on his Senate-attendance record on its head. Later in that debate, Rubio deflected a question about his personal finances by slamming supposed media bias, garnering one of his biggest applause lines of the night.
But Rubio has yet to clear one hurdle that move prove a headache for him at the fourth Republican debate and beyond: immigration.
Rubio's stance on immigration looks like a potential flash-point during the Tuesday night's Republican debate in Wisconsin.
One source familiar with the debate planning told Business Insider it could come up during questioning.
And fellow candidates might also make it an issue. Over the past several days, several candidates who will share the stage with Rubio have criticized the senator's support in crafting the bipartisan comprehensive immigration reform legislation that stalled in Congress in 2013.
"He was a co-author of the bill. It was a Rubio bill. It was a Rubio-Schumer bill," Sen. Rand Paul (R-Kentucky) said, referring to Rubio and Sen. Chuck Schumer (D-New York). "So, he does have to explain it. I think it will be a big part of things."
Real-estate mogul Donald Trump, meanwhile, has repeatedly criticized Rubio over his past support for a legal path to citizenship for immigrants living in the US without permission.
For his part, Rubio has slammed Trump for his supposed flip-flopping generally on immigration. And he has attempted to walk a fine line on the issue. He has said that in order for any immigration bill to pass through Congress, it needs to pass more stringent border-security measures, including increased border security agents, drones, and cameras.
"We have to prove to the American people that illegal immigration is under control. It's not good enough to just say we're going to pass a law that will bring it under control. People demand to see it," Rubio said earlier this year.
But as he has begun to rise in the polls, Rubio has also started to take a harder line on immigration-reform efforts.
Rubio raised eyebrows last week when he appeared to reverse course on a previous immigration stance, saying that if he were elected president, he would end the Deferred Action for Childhood Arrivals program that extends some protections to the young children of immigrants who come to the United States without permission.
Rubio had previously said that the program shouldn't be ended because it would create a sense of limbo for the children who have already benefited from the policy under President Barack Obama.
"DACA is going to end. And the ideal way for it to end is that it's replaced by a reform system that creates an alternative," Rubio said last week. "But if it doesn't, it will end. It cannot be the permanent policy of the United States."