Sen. John McCain's office said Monday evening that he would return to the U.S. Senate on Tuesday.

That follows the news last week that the Arizona Republican had been diagnosed with a brain tumor.

"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," the office said in a statement.



Health-care reform is an issue of critical importance for Republicans, who have campaigned for years on repealing and replacing the Affordable Care Act — also known as Obamacare.

But despite control of the House, the Senate and the White House, the GOP has struggled to push through a replacement plan, repeatedly hitting legislative roadblocks from members of their own party.

Republican action on the health care bill will also be important to the millions of Americans who receive their coverage through Obamacare.

Beyond that, investors are closely watching the progress of health-care reform because the GOP explicitly chose to address the issue before it took on taxes. That is, every setback for the Obamacare overhaul is seen as delaying the party's broader fiscal agenda.

This story is developing. Please check back for further updates.

— CNBC's Jacob Pramuk contributed to this report.