California billionaire Thomas Steyer put another $1 million into the super PAC he runs, a group that has backed Masschusetts Democrat Ed Markey's run for the Senate seat left vacant when John Kerry took the job of Secretary of State. Voters will go to the polls in the special election in less than two weeks.

According to a filing received Thursday night, NextGen Committee received $1 million from Steyer on May 23. The former hedge fund manager turned environmental activist has single-handedly bankrolled the super PAC, giving a total of $1.75 million to it this year. Steyer's drawn attention in the past for his state-level giving; he's previously donated more than $33 million to California causes and candidates.

SEE THE DATA: All of the outside spending in the special election

NextGen Committee has spent $792,000 this year--all of it in the Massachusetts Senate special election. The super PAC has backed Markey, a member of the House, and opposed both Rep. John Lynch--his opponent in the primary--and Gabriel Gomez, the Republican nominee.

The super PAC reported having just under $650,000 in the bank as of June 5.

The special election in Massachusetts has drawn more than $4 million in outside money, though most polls show Markey ahead in the traditionally Democratic-leaning state. The election will take place June 25.