Fox News has decided to stop running San Francisco billionaire Tom Steyer’s pro-impeachment TV ad, and Steyer is accusing the channel of breaching a contract.

The widely-viewed ad calling for the impeachment of President Donald Trump has been running nationwide for almost two weeks. It got a tweeted response last week from Trump himself, who called Steyer “wacky & unhinged” — apparently after seeing the ad on Fox & Friends, Fox News channel’s morning show.

Steyer purchased a second week of national Fox News advertising spots on Oct. 27, as part of an $11 million ad buy. But on Tuesday, Oct. 31, Fox News stopped running the ad, Steyer said Friday.

Fox News confirmed in a statement that the network had pulled the ad off the air.

“Due to the strong negative reaction to their ad by our viewers, we could not in good conscience take their money,” the channel’s co-president, Jack Abernethy, said.

The ad is still running on other Fox-owned television networks, Steyer’s group said.

“If Fox News is siding with Trump and trying to silence us, they must be afraid of what we have to say,” Steyer, a Democratic donor and philanthropist, said in a statement. “It shows no respect for democracy.”

In a letter to Fox on Friday, Steyer’s lawyers argued that not running the ad amounted to a breach of contract, and said the channel had given Steyer no explanation for why it had pulled the ad.

“The only plausible explanation seems to be that Fox News capitulated to political pressure from the Trump administration itself,” his lawyers write. “President Trump has threatened retaliation against broadcasters who provide him with negative coverage, and Fox News appears to have answered these threats with servility.”

The ad, which argues that Trump has obstructed justice and put the U.S. at risk of nuclear war, is also continuing to run on other TV channels, YouTube and Facebook. More than 1.6 million people have signed a petition associated with the ad calling for Trump’s impeachment, Steyer’s group said.