By Eugene Scott

CNN

(CNN) -- Sen. Elizabeth Warren praised her colleague John McCain Thursday, calling him "tough as a boot" the day after it was announced that the Arizona Republican had been diagnosed with a primary glioblastoma, a type of brain tumor.

McCain underwent surgery to remove a blood clot on Friday at Mayo Clinic Hospital in Phoenix. Lab results from that surgery confirmed the presence of brain cancer.

The Massachusetts Democrat spoke of McCain's tenacity during their trip to Afghanistan earlier this month.

McCain was "tough as a boot and ready to go at every moment," she told CNN's Phil Mattingly. "When you travel with John McCain, you get up early and you work until late at night because that's who he is."

Warren said she had no sense at the time that the 80-year-old, who had previously successfully battled skin cancer, was ill.

Despite being from opposing parties, Warren said she has been able to find some common ground with the 2008 Republican presidential nominee.

"He was really strong on breaking up the giant banks, things that pose a real risk to the economy and he didn't mind saying so," she said.

Warren praised McCain, who was a prisoner of war during Vietnam, for vocalizing that there is no "military-only solution" in Afghanistan.

"He was really pounding on the Trump administration to make sure that we had a full compliment of our diplomats," she said. "And that we need a strategy before we send more people into Afghanistan."

"There are places where we don't agree, but there are some key places where we did," Warren added.

Warren, a frequent critic of President Donald Trump, pounded him for expressing his frustration with Attorney General Jeff Sessions for recusing himself from the ongoing investigation into Russia's interference in the 2016 presidential election.

"I didn't think Jeff Sessions should be attorney general of the United States at all. I thought he was disqualified to begin with," she said. "He had been back in the 1980s turned down by this Congress for being too racist to be confirmed as a federal judge."

While Warren didn't want to see Sessions become the country's top law enforcement official, she said his recusal was the appropriate thing to do.

"Once he was confirmed, he followed the rules that are in place at the Department of Justice on recusal," she said. "And for Donald Trump to say, 'Gee, I had in mind to have an attorney general who would not follow the law' is just stunning. And it tells you once again that Donald Trump is out for exactly one thing, and that is to protect the skin of Donald Trump."

Warren graded Trump on his key campaign promise to "drain the swamp."

"Donald Trump promised repeatedly during the campaign that he would drain the swamp, so I want to hold him accountable for that," she said. "And it turns out that not only has he not drained the swamp, he has brought in -- we actually did one more count last night -- 193 corporate lobbyists, CEOs and executives of big, giant corporations and industry consultants."

"That's who he's bringing in every part of government and its having an impact on every part of government," added Warren. "That's why I give Donald Trump an 'F' on his promise to drain the swamp."