This story has been updated.

Sen. Marco Rubio (R-Fla.) vowed Wednesday to block any attempts by President Obama to expand U.S. diplomatic relations with Cuba as the Cuban-American senator quickly seized on the surprise developments and drew a sharp distinction between himself, the White House and potential GOP opponents in the 2016 presidential race.

In a series of appearances after the surprise announcement, Rubio repeatedly denounced Obama, while also stressing that he was "overjoyed" by the release of former prisoner Alan Gross and that he still hopes that the United States and Cuba can enjoy normal relations one day — but only if a democratically elected government is leading his father's homeland.

Addressing reporters during a hastily-arranged news conference in a nearly-empty U.S. Capitol, Rubio called Obama's decision "a victory for the oppressive Cuban government but a setback for the repressed Cuban people." He called Obama "the worst negotiator ever" to live in the White House.

"This entire policy shift announced today is based on an illusion," he added.

"The Cuban people — like all those oppressed around the world — they look to America to stand up for these rights, to live up to our commitment to the God-given right of every person, to life, liberty and pursuit of happiness." He said it is "unacceptable that the only people in this hemisphere that have not known democracy ... are the people of Cuba."

For years, critical comments by GOP lawmakers against Obama's foreign policy have been widely reported and absorbed but had little practical effect on the legislative or appropriations process. But on Wednesday, Rubio reminded reporters that he is poised to lead a Senate subcommittee on western hemispheric affairs — a perch that will give him direct oversight over America's policy in Cuba.

"I anticipate that we're going to have a very interesting couple of years discussing how you're going to get an ambassador nominated and how you're going to get an embassy funded," he said.

That threat bore the hallmarks of the late Sen. Jesse Helms (R-N.C.), who used a similar position on the Senate Foreign Relations Committee during the Clinton administration to block another Democratic president's attempts to revamp relations between the U.S. and Cuba.

Rubio, who is Catholic, acknowledged that his stance puts him at odds with Pope Francis, but he called on the pontiff "to take up the cause of freedom and democracy, which is critical for a free people."

Rubio will likely also face resistance from at least some fellow Republicans.

Sen. Bob Corker (R-Tenn.), who will chair the foreign relations panel next year, urged a wait-and-see approach: "There is no real understanding as to what changes the Cuban government is prepared to make," he said in a statement. "We will be closely examining the implications of these major policy changes in the next Congress."

And Sen. Jeff Flake (R-Ariz.), who traveled with two Democrats to Cuba on Wednesday to bring Gross home, strongly disputed Rubio's characterizations: "The policy that we’ve had in place for the past 50 years has done more in my view, in many’s view, to keep the Castro regimes in power than anything we could have done," he told reporters.

Flake dismissed Rubio's concerns that the Cuban government will severely restrict the movement of Americans who visit the country. "I’ve always said that if someone is going to limit my travel, it should be a communist, not my own government," he said. "It makes more sense for us to allow travel, that’s what we do. That’s what we do as Americans."

Rubio's first comments on the surprise announcement came Wednesday on the Fox News Channel, a cable network with a large audience of conservative viewers and closely-watched by GOP political operatives. He told the network that Obama's decision "does set a very dangerous precedent" and "puts a price on every American abroad. Governments now know that if they take an American hostage, they can get very significant concessions from the United States."

But Rubio dismissed suggestions that the surprise news on Cuba would accelerate or in any way influence his presidential ambitions.

"I am not discussing 2016 today, out of respect for the gravity and the importance of this issue," he said during his news conference. "I will just say on the issues that we can do here in Congress, we're going to do everything we can."