If they manage to push Johnson out — or if he resigns — the party would hold a leadership contest to choose his replacement.

Here's a look at who could rise if Johnson falls:

RISHI SUNAK, TREASURY CHIEF

Sunak, 41, is widely regarded as the brightest rising star in the party, the best known of the contenders to the public — and the bookies' favorite to succeed Johnson.

Sunak was thrust into the spotlight when he became treasury chief in early 2020, tasked with the unenviable job of steering the British economy through its worst economic slump on record due to the pandemic.

Sunak has dished out billions of pounds in emergency spending to help businesses and workers, and his pandemic policies have generally been seen in a positive light.

A big "Star Wars" fan, Sunak nurtures his personal brand with a slick Instagram account. Opinion polls have suggested that he is one of the most popular Conservative ministers among voters, though his elite education and past work for the investment bank Goldman Sachs and a hedge fund means some see him as out of touch with ordinary people.

He would be Britain's first prime minister who is not white. Born to Indian parents who immigrated to the U.K. from East Africa, Sunak attended the exclusive Winchester College private school, studied at Oxford University, and is married to Akshata Murthy, daughter of an Indian billionaire.