US Senator John McCain will miss a key vote on the Republican tax bill because of a viral infection, further narrowing the already-thin margin of support for the controversial plan.

The veteran Republican lawmaker, 81, will not return to Washington until January, his office said Sunday.

With McCain absent, the Republicans – who have a 52-48 majority in the Senate – can only afford one defection to pass the tax bill, with a vote expected in the coming days.

No Democrats or independents support the legislation in the Senate. It is expected to comfortably pass the House of Representatives.

The bill, which would slash corporate and individual taxes, has been criticized as primarily benefiting the wealthy.

It is President Donald Trump’s last shot at a major legislative accomplishment in his first year in office.

McCain, the 2008 Republican presidential nominee, was diagnosed in July with a brain tumor known as a glioblastoma, after doctors found a blood clot over his left eye during a routine checkup.