In a wide-ranging interview with CNN's Wolf Blitzer, the Florida Republican also gave his thoughts on Republican efforts to defund the president's executive action on immigration, and further explained why he won't support the administration's new Cuba policy.
Rubio, who released a new book "American Dreams" on Tuesday, said he's still deciding whether he thinks he can be more effective as president or as a senator under the new majority. He has already said he won't run for both offices in 2016.
With power players Mitt Romney and Jeb Bush now considered likely contenders, Rubio said "they're both credible and well-funded" candidates but argued there would still be room for his campaign if he decides to plow forward.
"I'm confident that if we decide to run for president ... we will have the funding and the resources necessary to credibly run a campaign and win," he said.
"But I understand that the longer you wait, the harder it becomes to do that," he added.
Bush and Romney have already been in active in talking with big-money supporters and securing financial resources, making it strategically more difficult for other potential contenders like Rubio to lock down support from the GOP's donor class.
Rubio, 43, will huddle with donors and supporters at a retreat he's hosting in Florida later this month, where plans could be formalized for 2016. He said he doesn't have a date in mind to make a decision, only saying he'll decide "soon enough so that you can build the kind of campaign it takes to win."
Pressed on whether that means "within the next few weeks," Rubio replied: "Yes, soon."
To combat President Barack Obama's executive action on immigration — which would allow up to 5 million undocumented immigrants to stay in the country temporarily -- Republicans on Capitol Hill are pushing a strategy to tie funding for the order to a must-pass spending bill for the Department of Homeland Security.
Rubio, however, questioned whether it was the right move, saying he's not in favor of "something that would be traumatic for the country or something that would put the nation in danger."
"I could support that, but ultimately if there's going to be a veto threat — as I'm sure there will be and the consequences of that is that we can't secure our nation -- that would be a real problem for a lot of people," he said. "For me included."
He added "if there was a way to do it that overcomes that obstacle, I would be all for it."
Rubio was among a bipartisan Gang of Eight senators who helped craft a comprehensive immigration reform package that passed in mid-2013
but failed to advance in the House.
He said he doesn't regret being part of the process but argued he wouldn't do it the same way again.
"We learned that you're not going to reform immigration law in America with...everything all in one big bill," he said. "I think that is true for multiple topics. I think that is especially true when it comes to immigration."
As he also lays out in his book, Rubio said the "only way" to address immigration reform will be through a sequence of bills, rather than one massive reform package -- with the first step being to better enforce current immigration law and with a pathway to citizenship as an ultimate goal of reform.
Following Obama's announcement last month
to loosen the U.S. embargo on Cuba, Rubio said he still plans to vote against any new funding for a U.S. embassy in Havana.
He also criticized the Cuban Interests Section in Washington — a diplomatic mission in the nation's capital — as "almost an exclusively intelligence gathering outpost."
"I mean, they conduct no diplomacy and there are a large number of people trying to get into Cuba," Rubio said. "The reason why our Interests Section is so busy is the large number of people trying to get out of Cuba."
The junior senator from Florida, whose parents emigrated from Cuba, said any policy change towards Cuba must be met with "reciprocal steps."
"If you want a better relationship with us economically, that should have been as a result of an opening politically on the island of Cuba," he said.