Republican presidential candidate Marco Rubio speaks at a pancake breakfast this week at a Veterans of Foreign Wars hall in Franklin, N.H. (Darren McCollester/Getty Images)

Sen. Marco Rubio (R-Fla.) was concluding his remarks about the merits of comprehensive immigration reform when he pointedly defended himself against conservative critics.

“Leaving things the way they are — that’s the real amnesty,” he said in April 2013, using one of the dirtiest words in the far-right lexicon to illustrate why he stood there as one of the bipartisan “Gang of Eight” senators unveiling a comprehensive immigration reform bill.

More than two years later, as a leading Republican candidate for president, Rubio must once again explain his membership in the group to a skeptical conservative base. After simmering in the background for months, Rubio’s past support for comprehensive reform — he has since renounced it — was litigated before a national audience during a Dec. 15 televised debate. In the time since, it has gained a deeper foothold in the daily campaign fray than ever before.

Talk-radio hosts and conservative writers have brought it up in unflattering ways. Meanwhile, one of Rubio’s chief rivals, Sen. Ted Cruz (R-Tex.), has been reminding Republicans of Rubio’s participation in the bipartisan effort.

At a time when Donald Trump’s harsh anti-immigration rhetoric has strengthened his bid for the Republican nomination, some conservatives believe the renewed spotlight on Rubio’s past could amount to a deadly blow. His aides and allies take a different view: He has put forth a clear and reasonable defense of his positions on immigration, in contrast to Cruz, who has not.

“There are a lot of conservatives who feel lied to by Rubio,” said conservative blogger Erick Erickson.

Rick Wilson, who works for a pro-Rubio super PAC called Baby Got PAC, said, “what this debate ended up producing on immigration is a lot more heat than light.”

“If folks were opposed to Marco Rubio before this, it was already baked into the cake,” Wilson said. He declined to say whether the PAC was preparing any advertisements on immigration.

Rubio will embark Monday on a three-day bus tour across Iowa, where Cruz has soared to the lead in recent surveys, and the Rubio campaign announced Saturday that Rep. Trey Gowdy (R-S.C.), the chairman of the House committee that investigated the 2012 Benghazi attacks, will join him there. The trip will be a good test of how much voter interest there is in Rubio’s work on the Gang of Eight, coming in a state where conservative activists play a dominant role.

In the first four debates, Rubio did not face the barrage of immigration questions and attacks he encountered in the fifth one. A moderator pointed out that he worked with Democrats on a bill, and Cruz, who voted against it, piled on.

“There was a time for choosing, as Reagan put it. Where there was a battle over amnesty, and some chose, like Senator Rubio, to stand with Barack Obama and Chuck Schumer and support a massive amnesty plan,” Cruz said. “Others chose to stand with Jeff Sessions and Steve King and the American people and secure the border.”

Rubio no longer supports comprehensive reform; he walked away from it after the bill passed the Senate but was derailed in the GOP-controlled House by a conservative uproar. He still backs a path to citizenship for undocumented immigrants — but only after illegal immigration is curtailed and the current system is modernized.

“I think we need to deal with immigration once and for all. It’s been confronting the country for 30 years,” Rubio told reporters in Iowa two days after the debate, adding later: “It’s not going to be able to be comprehensive. It’s going to have to be done in stages.”

Throughout his campaign, Rubio has told voters that he knows from firsthand experience that a comprehensive approach will not work politically.

Rubio shrewdly put Cruz on defense in the debate, pointing to an amendment Cruz submitted that would have granted legal status but not citizenship to undocumented immigrants. Cruz, who opposes legalization now, claims the amendment was a poison pill. But he’s been surrounded by questions about his true intentions at the time.

Rubio and his backers see a chance to advance two arguments: Rubio and Cruz are not so different on immigration, and that Cruz is slippery, not the rock-ribbed conservative he claims to be.

But on influential conservative talk radio, a different conclusion has emerged amid the heightened focus on immigration.

“At the end of the day when people go vote, people are going to remember, of the two, it was Marco Rubio that was a member of the Gang of Eight and Ted Cruz that wasn’t, and that’s as complicated or simple as it’s going to end up being,” said Rush Limbaugh on a recent show.

Some Republicans say immigration could lead to broader worries for Rubio stemming from a sense that the GOP leaders abandoned their core principles when they embraced comprehensive reform.

“I do think that the immigration issue hurts him more than Ted Cruz because he wasn’t able to get” a law passed and “Democrats were able to use his name,” Erickson said.

Cruz and his allies are prepared to draw a sharp contrast with Rubio in the coming weeks. Even before the most recent debate, a super PAC backing Cruz released a radio ad asking: “Other than his Gang of Eight amnesty bill, can anyone think of anything Marco Rubio’s ever done?”

The next televised debate is scheduled for Jan. 14 in South Carolina. Fox Business Network will host the debate, which means economic issues will probably be a major focus. To Rubio backers, widening the discussion in the primary well beyond immigration would be good news for the first-term senator from Florida.

“At the end of the day there is going to be a lot of debate about this, but there’s going to be a debate about a lot of other broad and consequential issues,” Wilson said.