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By Andrea Mitchell and Jon Schuppe

Secretary of State John Kerry said Monday that despite opposition to historic agreements with longtime rivals Cuba and Iran, time will prove that the United States did the right thing.

Speaking just hours after the United States formally restored full diplomatic relations with Cuba, Kerry told NBC News' Andrea Mitchell that Republican vows to cut off financing for America's new embassy in Havana and to refuse to approve a Cuban ambassador "just doesn't make sense."

"To not be able to meet with more people in Cuba, to know what is going on, is a huge cut off of opportunity," Kerry said. "I just think it's cutting off your nose to spite your face. And it's a shame."

See more of the interview on MSNBC's "Andrea Mitchell Reports" at 12 p.m. ET Tuesday.

Kerry, fresh from a meeting with his Cuban counterpart, Foreign Minister Bruno Rodriguez, answered criticism of Cuba's human rights record by arguing that the United States would be able to more effectively address its concerns with diplomats able to engage officials and the public.

Those issues have been allowed to fester for the past half-century, Kerry said.

"We want to engage, and when you get to that point, that’s what begins to break down the barriers," he said.

Congressional opponents, and some Republican presidential candidates, have vowed to keep a 53-year-old trade embargo in place, and to roll back the advances in diplomatic relations.

But Kerry predicted that by the time President Obama's successor takes office in 2017, it would be virtually impossible to change course.

"I believe the president has taken an irreversible step," Kerry said.

Turning to Iran, Kerry said efforts to scuttle a deal that would lift economic sanctions in exchange for curtailing the country's nuclear program would peter out as the American people learned that the benefits far outweighed the costs.

"This is the best opportunity to avoid conflict and to know what their program is doing," he said. "And if, at the end, all those inspectors and all the rest of our knowledge of their program shows us that they are trying to step outside it, we have every option available to us then that we have today. We lose nothing."

One opposition group, called Citizens for a Nuclear Free Iran, has said it plans to spend at least $20 million on advertisements and a letter writing campaign to persuade Congress to reject the compromise, reached last week between Iran and the so-called P5+1 — the five permanent members of the United Nations Security Council, plus Germany.

Kerry said he planned to counter that resistance "with common sense, with the truth" — an effort that will begin with visits to Capitol Hill this week.

"They just want to kill it, but they don’t have an alternative," Kerry said. "And the status quo is unacceptable, because in the status quo, Iran was marching full square toward having a bomb."

Finally, Kerry defended fellow Vietnam veteran and former Senate colleague John McCain, whose heroism as a prisoner of war was called into question last weekend by presidential candidate Donald Trump.

McCain's record of military service, and conduct as a prisoner of war, "is the definition of heroism," Kerry said. "To not understand that is to neither understand what it meant to serve, what it means to serve, or what heroism really is."