An "increasingly frail" Sen. John McCain remained hospitalized Thursday in an effort to manage side effects of his treatment for brain cancer.

CNN cited sources in the U.S. Senate saying the Arizona Republican was "looking increasingly frail and said he has not spoken up in recent GOP meetings the way he had before."

Sen. Lindsey Graham, R-S.C., and a close friend of McCain's, tried to sound a hopeful note.

"I feel pretty good about the way the treatment is affecting his underlying cancer," told CBS News. "But the treatment has a downside. So he’s tryin' to get rested up. I’m very confident he’ll come back.”

Still, McCain has missed all votes in the Senate this week and his health is increasingly a concern as Republicans in the House and Senate close in on a final deal for $1.5 trillion in tax cuts.

The GOP is trying to maintain a fragile coalition, especially in the Senate, that could give President Donald Trump his first significant legislative achievement. McCain's support, however, is a key piece of making that happen.

It remained unclear Thursday how long McCain would be hospitalized at Walter Reed Medical Center outside Washington.

In announcing the senator's hospitalization on Wednesday, his office had said: “Senator McCain is currently receiving treatment at Walter Reed Medical Center for normal side effects of his ongoing cancer therapy. As ever, he remains grateful to his physicians for their excellent care, and his friends and supporters for their encouragement and good wishes. Senator McCain looks forward to returning to work as soon as possible.”

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