Veteran US Senator John McCain will return to the chamber Tuesday after a brain cancer diagnosis for a key vote to launch a process to repeal and replace Obamacare, his office announced.
"Senator McCain looks forward to returning to the United States Senate tomorrow to continue working on important legislation, including health care reform, the National Defense Authorization Act and new sanctions on Russia, Iran and North Korea," a statement read. 
Republicans hold 52 of the Senate's 100 seats. With all Democrats opposed, President Donald Trump can afford just two defectors on the health care reform vote, making McCain's vote critical.