Expressing deep concern about the impending departure of Secretary of Defense Jim Mattis, Sen. Ben Sasse said Mattis was "giving advice the president needs to hear."
Both Sasse and Rep. Don Bacon described Thursday's announcement of Mattis' impending retirement after his sharp division with President Donald Trump over the president's decision to withdraw U.S. troops from Syria as "a sad day."
Sasse is a member of the Senate Armed Services Committee.
Bacon is a retired U.S. Air Force brigadier general.
"Mattis rightly believes that Russia and China are clear adversaries and that we are at war with jihadists across the globe who plot to kill Americans at home," Sasse said.
"Isolationism is a weak strategy that will harm Americans and America's allies," the Republican senator said. "Radical Islamic jihadists are still at war with us and ISIS is not gone."
Bacon said the country "will miss (Mattis') wisdom, experience and tremendous leadership."
Mattis is "greatly trusted by all our allies and respected by our adversaries," the Republican congressman said.
Sen. Deb Fischer, also a Republican member of the Senate Armed Services Committee, tweeted high praise for Mattis, describing him as "a patriot."
"The American people are forever grateful for his long record of service to our country," Fischer said. "He has my enduring thanks for all he has done to keep us safe and secure."
Former Secretary of Defense Chuck Hagel said Mattis "had no choice but to resign" and that is "a loss for our country."
[Watch: Hagel tells CNN Trump not fit to be commander-in-chief]
"The continued exit of competent and experienced leaders in our government is dangerous," Hagel said. "Our government is not a one-man show."
"America is headed for a very difficult and messy next two years," Nebraska's former Republican senator said.
"We need for Congress to play its important and constitutional oversight role," Hagel said, "and be a real partner to the presidency."