David Brock, the founder of liberal website Media Matters, reportedly suffered a heart attack Tuesday.
According to NBC News editor Bradd Jaffy, Brock's chief of staff confirmed the news.
Brock, 54, founded the liberal watchdog in 2004 and has been an active and vocal opponent of Republican candidates and politicians. During last year's presidential election cycle, he used his position to promote Democrat Hillary Clinton.
Last week, he offered $5 million "to anyone with [President Donald] Trump's complete, legally obtained tax returns."