WASHINGTON — Loud but hardly universal catcalls from Republicans underscored the obstacles and opportunities ahead as U.S. and Cuban leaders announced an opening of embassies in Havana and Washington and a resumption of diplomatic relations severed the year President Barack Obama was born.

Obama also called on Congress to lift the economic and travel embargoes that the U.S. has used for decades in an attempt to force Cuba’s leaders toward democracy. Obama has partly eased those restrictions on his own, but continued opposition from many Republicans and some Democrats makes it unlikely that lawmakers will fully revoke those bans quickly.

Labeling the moment “a choice between the future and the past,” Obama on Wednesday revealed the latest steps in a half-year of rapid-fire improvements in relations between two nations that lie 90 miles apart but have spent nearly six decades separated by light years diplomatically and economically.

“There are those who want to turn back the clock and double down on a policy of isolation,” Obama said in the White House Rose Garden. “But it’s long past time for us to realize that this approach doesn’t work.”

In an exchange of notes with Cuban President Raul Castro, the two governments said that on July 20 they will open embassies in each other’s capitals that have been shuttered since 1961. That is when President Dwight Eisenhower broke relations with the communist regime of Raul’s brother, Fidel Castro, setting the tone for decades of Cold War hostility that included failed U.S.-backed efforts to overthrow the island nation’s leaders.