Opponents of President Obama’s diplomatic opening toward Cuba began plotting for the long road ahead to block the administration’s new policy, focusing on areas where congressional consent is necessary.

The most likely targets are funding for new diplomatic operations in Havana as well as the requirement for Senate confirmation of an ambassador, and while the issue has divided Republicans, key conservatives with long anti-Castro records occupy powerful positions in Congress and could thwart Obama’s overtures toward Cuban President Raúl Castro.

The GOP leaders are throwing their weight behind the efforts of three Florida Republicans of Cuban heritage, Sen. Marco Rubio and Reps. Ileana Ros-Lehtinen and Mario Diaz-Balart.

“I don’t think conditions have changed much. I haven’t seen much evidence that anything’s changed,” Sen. Mitch McConnell (R-Ky.), the incoming majority leader, said in an interview Thursday, suggesting that the regime’s human rights abuses had led him to support his Florida Republican colleagues. “They understand this issue pretty thoroughly. I’m at least persuaded that Marco’s right about this.”

Their staffs have begun scouring pertinent laws to determine whether there are ways to impede the new financial avenues to commerce with the island nation. In particular, GOP aides are focusing on portions of the 1996 law that tightened the embargo against Cuba, whether the president’s decision to allow U.S. credit and debit cards in Cuba would violate sections of the law, commonly referred to as the Helms-Burton Act.

Derailing Obama’s initiatives will not be easy, and it could take many months, if not years, to play out as the new Congress takes up the annual funding bills for federal agencies and other oversight actions.

“In my mind, we intend to use everything at our disposal to address this in the most positive way possible,” Rubio said Thursday at a packed news conference in the Miami office of Ros-Lehtinen, promising to “look at all of our options.”

Just four years into his Senate tenure, Rubio is already the third-ranking Republican on the Foreign Relations Committee and expects to chair a subcommittee overseeing Western Hemisphere issues. He will be able to hold hearings and call witnesses to try to shape the issue, possibly as he mounts a campaign for the 2016 Republican presidential nomination.

However, in a sign of the lack of ideological conformity, Sen. Rand Paul (R-Ky.), a member of the full committee and Rubio’s subcommittee, said Thursday that the Cold War-era policy toward Cuba “just hasn’t worked” and normalizing relations is “probably a good idea.”

That divide could turn any potential confirmation hearings for a new ambassador into a brutal fight.

Rubio is joined in his strident opposition by Sen. Robert Menendez (D-N.J.), a Cuban American who is the outgoing chairman and who expressed outrage at Obama’s move.

Their views would be countered by Paul and Sen. Jeff Flake (R-Ariz.), a conservative who flew to Havana on Wednesday to help pick up freed American prisoner Alan Gross.

And the incoming chairman, Sen. Bob Corker (R-Tenn.), has so far been noncommittal, saying Wednesday that the committee “will be closely examining the implications of these major policy changes in the next Congress.”

The effort to block funding for the new policy will fall to Diaz-Balart, a senior member of the House Appropriations Committee. He repeatedly denounced Obama, Secretary of State John F. Kerry and former secretary of state Hillary Rodham Clinton for negotiating the prisoner exchange despite public pronouncements that they would never exchange Gross’s freedom for the return of three Cuban spies.

“Yesterday they did exactly what they claimed they would never do,” Diaz-Balart said Thursday at the Miami news conference with Rubio and Ros-Lehtinen. “It shows a deep level of cynicism.”

As a senior member of the subcommittee that funds the State Department, Diaz-Balart will have a hand in approving diplomatic budgets. House Speaker John A. Boehner (R-Ohio) and McConnell are increasingly preaching to their rank-and-file Republicans that these annual spending bills are where they can advance conservative goals by attaching provisions that reduce or eliminate funds for projects, or by attaching policy riders that specifically forbid federal agencies from taking actions.

Just this week, a sweeping spending bill passed Congress that included a controversial provision to alter banking laws on risky trades, serving as the sort of example of what Diaz-Balart could try to do on Cuba policy as those bills head through Congress next summer. Diaz-Balart has a key ally in the Senate — Sen. Lindsey O. Graham (R-S.C.), a staunch opponent of the new policy who will lead the subcommittee approving the State Department’s budget.

Still, Obama could veto those individual spending bills if they include Cuba restrictions he finds objectionable, which would set up a showdown over whether Republicans are willing to shut down portions of the government over the diplomatic openings to Cuba. There are some suggestions that existing property in Havana — known as the U.S. Interests Section — could be converted into a temporary embassy without requiring new funding.

Ros-Lehtinen, a 26-year veteran of Congress who once chaired the House Foreign Affairs Committee, serves as the political godmother of the anti-Castro movement on Capitol Hill.

On Thursday, she hosted Rubio and Diaz-Balart alongside family members of the “Brothers to the Rescue” pilots killed in 1996 when Cuban jets shot down their aircraft in the waters off the island.

In Spanish, she said that Obama’s moves “destroy the dreams of millions of Cubans who’ve waited for half a century for their liberty. Obama insulted the Cuban American community that wants freedom and democracy for Cuba.”

Rubio added a veiled attack on Ben Rhodes, a deputy national security adviser who once wrote Obama’s foreign policy speeches, noting Raúl Castro’s claim that Cuba “agreed to nothing” as part of the deal that Rhodes helped broker. “This is the kind of deal you get when you send your speechwriter to negotiate with a tyrant,” he said.

McConnell predicted a majority of Senate Republicans would side with Rubio.

“Even though I understand the argument that engagement may bring about changes, in this particular country — right next door to our country — I think the arguments of people like Menendez and Rubio are compelling,” McConnell said.

Obama stunned McConnell when he called him on Tuesday night to inform him of the move.

“It was such a surprise, we haven’t talked about this issue in quite a while. . . . To be perfectly candid with you, I hadn’t thought about Cuba policy in quite a while, there just hasn’t been anything on the agenda lately.”