Corker has grown increasingly unnerved about Trump's sharp attacks against NATO, raising alarms that the President's rhetoric and actions have "destabilized" a key part of the post-World War II order.
"It's palpable," Corker said when asked if Trump has damaged US standing around the world. "The concern people have is to our reliability."
Corker added that "I do worry" Trump is trying to "tear apart" NATO.
"I believe that America's leadership around the world has made the world safer for Americans and has made the world a better place," said Corker, a second-term Tennessee Republican. "And when I see that leadership diminishing, and us trying to break apart alliances that we created, it troubles me."
But Corker, who is retiring at year's end, is a rare Republican who publicly voices his concerns with Trump's moves .
"I think there are ways of communicating with your friends and sometimes it feels like we punch our friends in the nose and hold our hand out to people who are working strongly against us, like Russia, and Putin," said Corker, who met with foreign leaders last week in the Nordic and Baltic regions.
The sharp words came as Trump kicked off his trip to NATO on Wednesday morning by telling NATO President Jens Stoltenberg that, "Germany is a captive of Russia." Trump went on to complain that the United States is expected to "defend them against Russia," despite Germany making "billions of dollars" in energy payments to Moscow.
"I think it's something that NATO has to look at," Trump said, the latest in a series of attacks he's launched against NATO. "Germany is totally controlled by Russia."
The comments prompted an avalanche of criticism from top Democrats, who called the remarks a "disgrace" and an "embarrassment" and demanded Trump apologize. Several Republicans, like Sens. Chuck Grassley and John Kennedy, defended Trump, saying he has the right to express his concerns.
Corker said he agreed with Trump's underlying concerns with Germany's reliance on Russian energy and with pushing NATO countries to spend more on their defense.
But Corker said: "You can express yourself without trying to tear down an alliance that's been important to the security of Americans."
US allies, he said, no longer feel like the country is committed to NATO.
"There's a significant concern that people don't feel like we're committed to NATO," Corker said. "Again, you can express differences, without giving indication that you think NATO is a terrible alliance that's been bad for America. It's been great for America. ... I mean, those of us in the world and the west that care about capitalism and free enterprise and free trade, should want to strengthen those relationships, not to tear them apart."