ABC News' chief anchor, George Stephanopoulos, asked Sasse, R-Neb., about Trump's tweet Saturday attacking a federal judge who temporarily blocked Trump's immigration order as a "so-called judge."
"I'll be honest - I don't understand language like that," Sasse said in a "This Week" interview that aired Sunday. "We don't have so-called judges. We don't have so-called senators. We don't have so-called presidents. We have people from three different branches of government who take an oath to uphold and defend the Constitution. And it's important that we do better civics education for our kids."
Stephanopoulos also asked Sasse about Trump's comparing the U.S. with Russia under Putin. In a Fox News interview with Bill O'Reilly that is to air Sunday before the Super Bowl, Trump said he respects Putin. When O'Reilly said, "Putin is a killer," Trump responded, "We got a lot of killers. What, you think our country's so innocent?"
Sasse said that he has seen only short clips of the interview and that "there may be a broader context."
But he added, "I don't know what the president is trying to do with statements like he allegedly has on O'Reilly."
"Let's be clear," Sasse said. "Has the U.S. ever made any mistakes? Of course. Is the U.S. at all like Putin's regime? Not at all."
"There is no moral equivalency between the United States of America, the greatest freedom-loving nation in the history of the world and the murderous thugs that are in Putin's defense of his cronyism," Sasse said. "There's no moral equivalency there."
Stephanopoulos pressed, "Are Senate Republicans in the same place as President Trump on the issue of Russia?"
Sasse responded, "I don't understand what the president's position is on Russia, but I can tell you what my position is."
"Russia is a great danger to a lot of its neighbors, and Putin has, as one of his core objectives, fracturing NATO, which is one of the greatest military alliances in the history of the world," Sasse said. "And so Putin is a mess."
Sasse also said Trump's executive order on immigration was rolled out in a "clunky" way but praised him for focusing attention on the nation's borders.
"We haven't taken borders seriously enough, and we haven't done enough vetting of a lot of folks trying to come to the U.S., especially from nations that have failed states," Sasse said. "I think the administration has taken some important steps to improve upon the clunkiness of the initial weekend" after the executive order was signed.
Sasse praised Trump's nomination of federal Judge Neil Gorsuch for the Supreme Court.
"Judge Gorsuch is a rock star, and I applaud the president for this pick," Sasse said.
Stephanopoulos noted that Sasse was critical of Trump during the campaign and wrote in now-Vice President Mike Pence's name for president.
"From what you've seen over the first two weeks, has that increased your confidence in the president's leadership?" Stephanopoulos asked.
Sasse dismissed the question, saying all Americans could use a civics lesson to get the country through this fractured political environment.
"Every American, regardless of who you voted for - if you voted for Donald Trump, Hillary Clinton, Donald Duck, I don't care - we should all hope that the president does a good job, that he's surrounded by wise counselors, that he advances U.S. interests, and we should vigorously debate policy differences," Sasse said.