Rubio has long maintained that he would eventually end the White House program that shields some undocumented immigrants who entered before their 16th birthday from deportation, but he has been less clear about when that would happen.
Rubio told CNN's Jake Tapper on "The Lead" on Thursday that he would revoke the Deferred Action for Childhood Arrivals order immediately, a departure from past comments that suggests it would not take effect until deeper into his presidency.
"I said that DACA has to go away and that it will," Rubio told Tapper. "I will on my first day in office get rid of it because it's unconstitutional."
That stands in contrast to a prominent interview with Fusion that has been seen as Rubio's stance on the issue for the duration of the campaign.
"I don't think we can immediately revoke that," Rubio told anchor Jorge Ramos last April. "It cannot be the permanent policy of the United States, but I'm not calling for it to be revoked tomorrow, or this week or right away."
When asked about the apparent shift, Rubio communications director Alex Conant downplayed the senator's new promise to "get rid of it" on his "first day in office."
"He has always said that DACA is unconstitutional and that it needs to end," Conant told CNN. "As President, he'd stop people signing up for the program immediately."
Rubio diverges sharply from the Republican base given his accommodating posture on immigration, one of the most explosive issues in the Republican field. In addition to his position on DACA, Rubio also is open to a path to citizenship for those who came here illegally, a stance pilloried by GOP rival Ted Cruz in a series of attack ads.
Rubio has consistently said that a separate deferred action program for undocumented parents, known as DAPA, can be ended on his first day in office. He has also said that he would stop signing up new individuals for deferred action. But Rubio has conveyed a different message when it comes to younger immigrants who came without papers as children.
Cruz's campaign, which still envisions Rubio as a long-term political threat, sees the apparent shift as a reason to pounce.
"Marco knows he cannot win with his record on amnesty so he either has to change his position to deceive voters of his real record or distract voters from discussing it at all by misconstruing Cruz's record instead," said Cruz spokeswoman Catherine Frazier in a statement.