Updated at 11:48 a.m.
Rubio's Senate legislative resume is dominated by a sweeping immigration reform bill that he once pushed, but no longer backs. There are other areas -- national security spending, for instance -- where has also recalibrated his position.
It's common for major presidential candidates, and even political parties, to have to address position changes. Political climates and orthodoxies can change rapidly. And some Rubio opponents are feeling even more heat than he is on this front right now.
But fending off flip-flopping charges could be especially thorny for Rubio, some Republicans say, since he is already defined heavily by immigration -- an issue where his position now is not what it was as recently as 2013.
Mark Meckler, a leading tea party activist, said that whenever there is suspicion about whether Rubio's views have changed, it could remind conservatives about his push for comprehensive reform and then his swift move away from it.
"I think it amplifies the immigration stuff," said Meckler.
On Wednesday, Rubio told Charlie Rose of CBS News that he would not have ordered the Iraq invasion knowing what the country does now: that there were no weapons of mass destruction there.
"Not only would I not have been in favor of it, President [George W.] Bush would not have been in favor of it," said Rubio at the Council on Foreign Relations in New York, directly answering a question that has tripped up likely rival Jeb Bush in interviews and other appearances.
But in late March, when Rubio was asked by Fox News whether it was a mistake to go to war in Iraq, he responded: "I don't believe it was. The world is a better place because Saddam Hussein doesn't run Iraq."
Of course, Rubio's campaign denies that the two positions are inconsistent. The question he received at CFR was different than the one he fielded on Fox. Still, his answers appeared to express different overarching takes on whether the invasion was justified.
In a Thursday interview with Concord News Radio, Rubio explained why he does not think his answers were inconsistent.
"It was not a mistake in the sense that the president made the right decision based on what he believed and had reason to believe that time," he said in the interview, which was posted online by Buzzfeed.
Fergus Cullen, a former New Hampshire Republican Party chairman, said that Iraq is a "real thorny" issue for all Republicans, "not just Rubio."
"If there was a Facebook relationship status, it would say 'It's complicated,'" said Cullen.
Craig Robinson, former political director of the Iowa Republican Party, said that he does not believe Rubio has developed a reputation as a flip-flopper. But he added a word of caution: "If this becomes a trend ... then it becomes more problematic."
On immigration, Rubio was part of a bipartisan group of senators who tried to remake the nation's laws in 2013 through a bill that included a path to legal status for undocumented immigrants. The bill passed the Senate but was killed by conservatives in the House.
Rubio walked away from the proposal, and now backs a piecemeal approach that begins with border security and enforcing current laws.
“We can’t fix immigration in one massive piece of legislation,” Rubio said at a recent house party in New Hampshire. “I know that because I was part of an effort that tried. It doesn’t work.”
Rubio is not the only candidate who has to explain himself on immigration or other big issues. Former Wisconsin governor Scott Walker, a likely opponent, said in one setting that it "makes sense" to grant citizenship to undocumented immigrants. But in another, he said: "I don't believe in amnesty."
Sen. Rand Paul (R-Ky.), who is running for president, has faced questions about his shifting position on foreign aid and a border fence, to name just a couple of things.
More broadly, the Republican Party as a whole has shifted to the right on national security in recent years, becoming more aggressive.
But Rubio, still a new candidate for president, is starting to come under greater scrutiny. And so are the differences between what he is saying now and what he said a few years ago.
Rubio said in his CFR speech that his "first priority" as president would be to "adequately fund our military." He has been running on a staunchly hawkish platform. He introduced a budget amendment earlier this year to beef up defense spending without offsets in other areas.
But in the past, he has called for cuts in other areas to offset a boost in spending.
"To lift the sequester we must find a real, lasting solution to the true cause of our growing national debt: the unsustainable path of important programs like Medicare and Social Security," Rubio said at the American Enterprise Institute in 2013.
Asked to explain the shift on national security spending, a Rubio spokesman did not immediately respond.
Cullen said that Rubio is still relatively new to most Republican voters and that they don't have a deep knowledge of his policy history, making it unlikely that he is seen as a flip-flopper by many voters.
"I think Rubio is not well-enough defined in voters' minds to be drawing nuanced views about him," said Cullen.
That may be true. But when they get to know Rubio better, discrepancies both small and large could come back to haunt him.