WASHINGTON - The shift in U.S. policy toward Cuba triggered fierce partisan warfare Wednesday in Congress, as some Republicans vowed to take strong steps to block the changes.

"I am committed to doing everything I can to unravel as many of these changes as possible," said Sen. Marco Rubio of Florida, a view many top Republicans shared.

Among the possible strategies: They could refuse to end the economic embargo of Cuba, block funding for a new embassy, or stall or defeat an ambassadorial nomination.

President Obama and Cuban leader Raul Castro agreed Wednesday to free American aid worker Alan Gross on humanitarian grounds, while three Cubans convicted of spying went back to their country. The United States will now explore establishing an embassy in Cuba for the first time in 53 years, and ease travel and trade restrictions.

Leading the opposition Wednesday were lawmakers with close ties to Cuba, notably Rubio, the son of Cuban immigrants.

Obama's action does not end the economic embargo against Cuba, since that's up to Congress, which in the 1996 Helms-Burton Act toughened travel and economic restrictions against the island nation. The act said the embargo cannot be lifted until Cuba becomes democratic and its leaders do not include Raul Castro or his brother Fidel.

Obama pledged to work with Congress.

He's going to run head-on into a furious Rubio, who next year is expected to be chairman of the Senate's Western Hemisphere subcommittee, which considers legislation dealing with Cuba policy. Senate Republican leader Mitch McConnell of Kentucky, who will be majority leader next year, told the Associated Press he supports Rubio's position.

"This Congress is not going to lift the embargo," Rubio said flatly. Rubio is also mulling a run for the 2016 GOP nomination for the presidency.

Any negotiation will take some time, predicted Sen. Ben Cardin (D., Md.), a senior Foreign Relations Committee member. "It's too early to predict what will happen," he said.

The money dispute could begin early next year. If Obama requests funds to build an embassy, a Republican-led Congress would need to approve the request.

"I will do all in my power to block the use of funds to open an embassy in Cuba," tweeted Sen. Lindsey Graham (R., S.C.), who's slated to become chairman of the Senate Appropriations Committee's foreign operations subcommittee.

The Senate also would have to confirm an ambassador nominee, and almost any Obama pick would be subject to brutal scrutiny.