After an outpouring of support from across the political spectrum over his brain cancer diagnosis, Sen. John McCain (R-Ariz.) promised his “sparring partners in Congress” that he would return soon.

“Stand-by!” he tweeted on Thursday.

The diagnosis did not appear to sway the veteran Arizona senator, who subsequently criticized President Donald Trump for canceling a covert U.S. program to arm moderate rebels in Syria. The administration is “playing right into the hands of Vladimir Putin,” McCain said in a statement issued by his office.

If his health does end up preventing McCain from returning to Congress, however, Arizona law maintains that the state’s governor would choose his replacement.

That person would need to be of the same political party as the person who is vacating the seat ― in this case a Republican ― and would serve until the next general election in 2018.

“The person seeking the so-called McCain seat would serve out the rest of the term,” Arizona state elections director Eric Spencer told The Arizona Republic, meaning that whoever was elected in that election would not serve the traditional six-year term but only the duration of McCain’s until 2022.

If McCain resigns within the year, Arizonans would have two separate Senate contests on their ballots in 2018. Jeff Flake, the state’s junior senator, is also up for re-election next year, and is considered one of the most vulnerable Republican senators during that election cycle.

If McCain opts to stay in the Senate, but his health forces him to remain absent from the body, the implications could loom large for Trump and pose problems for the Republican legislative agenda in Washington.

The Arizona senator’s absence has already had an impact on the legislative calendar. Senate Majority Leader Mitch McConnell (R-Ky.) was forced to delay a critical vote on the Senate health care bill after McCain announced he had undergone a surgery to remove a blood clot from above his left eye last week.

Without McCain, who indicated he would at least vote to proceed to debate the legislation, McConnell’s task of securing the support of holdouts becomes much tougher. Republicans would functionally hold just a one seat majority in Senate, not two as they do now, with Vice President Mike Pence able to cast tie-breaking votes.

In the face of uniform Democratic opposition, McConnell can already only afford to lose two Republican votes on a Senate health care bill. If McCain were unavailable to vote on that and other key legislation, however, McConnell would face additional difficulties uniting his caucus and may even be forced to seek Democratic votes.

One fewer solidly Republican vote could also seriously imperil legislation concerning tax reform and infrastructure, two items on Trump’s agenda that McConnell has said he wants to move to next. Tax reform, in particular, looks like a potentially even more difficult task than the endangered health care bill.