Sen. Marco Rubio, R-Fla., said Wednesday that he would oppose any candidate to be the next ambassador to Cuba until the Castro regime stops imprisoning and oppressing democracy activists on the island, activity he says has stepped up since Obama's December announcement that he wants to normalize relations.

Rubio declared his intentions just minutes before President Obama was expected to announce the establishment of embassies in Havana and Washington.

"It is time for our unilateral concessions to this odious regime to end," he said in a statement released Wednesday morning.

Obama and Secretary of State John Kerry will announce the opening of embassies Wednesday morning, a major step in normalizing relations between the two countries. The announcement is being made a few weeks after the administration dropped Cuba from its list of terrorist-sponsoring nations — Cuba had insisted on that change before allowing embassies to open.

The move has already kicked up a fight between proponents of the president's policy to engage with Cuba and those like Rubio, as well as Sen. Ted Cruz, R-Texas. Both are Cuban-Americans who argue that re-establishing diplomatic and economic ties will only further hurt the Cuban people because the Obama administration is conceding too much without demanding democratic reforms.

For example, Rubio said the Obama administration has done nothing to secure the return of U.S. fugitives the Castro regime is harboring in Cuba, settle outstanding legal claims to U.S. citizens for properties confiscated by the regime, or secure "unequivocal" assurances that our diplomats will be able to travel freely throughout Cuba and meet with dissidents.

In June, Rubio warned Kerry that the U.S. government needs to make these issues a priority in its negotiations with Cuba, and if it did not, he would oppose the confirmation of any nominee to be U.S. ambassador.

The White House could instead choose to sidestep the fight by not naming an ambassador and allowing the embassy to function without one. There is already a small U.S. staff at the U.S. Interests Section in Havana, effectively an unofficial embassy, that could continue to operate without a formal ambassador.

Any Obama ambassador nomination to Cuba would likely further poison relations with the Senate in the middle of an already tense environment over nuclear negotiations with Iran, and broader disagreement over many other of Obama's foreign policy choices.