The top super PAC supporting Hillary Clinton's prospective 2016 presidential campaign is joining in a new effort to defend her from criticism of her handling of the killings of four Americans in Benghazi, Libya, last year.

In an e-mail set to go out to supporters, Ready for Hillary PAC forwards an op-ed from Media Matters founder David Brock hitting back against GOP critics of Clinton's actions.

Brock has written a new book titled "The Benghazi Hoax," which the e-mail also promotes.

"It is vital that we get the truth out, and with your help, I know we will," the PAC says in the e-mail, an advance copy of which was shared with Post Politics.

Brock also announced an ad buy promoting the book on Fox News this week but later said that Fox has declined to air the ads, which are highly critical of the channel's coverage of the attack on the U.S. Embassy in Benghazi that killed U.S. Ambassador Christopher Stevens and three other U.S. citizens.

Fox has not yet commented on whether or why the ads were spiked.

Republicans have seized on the Obama Administration's actions before the Benghazi attack and its statements afterward, alleging mismanagement and a cover-up. The controversy was enough to result in then-United Nations Ambassador Susan Rice pulling her name from consideration to succeed Clinton as secretary of state. John Kerry was later nominated and confirmed.

Since then, Republicans have turned their attention to Clinton, who at a hearing after the attack exclaimed "What difference does it make?" when asked what would have triggered the attack on the mission in Benghazi.

Here's the text of the e-mail from Ready for Hillary PAC:

Dear [FIRSTNAME|Friend],

It’s only 2013, and already Republicans and their allies in the conservative media are desperate to attack and defame Hillary, hoping that their lies will dissuade her from running for president. This past May, Karl Rove’s Super PAC, American Crossroads, already produced an ad attacking Hillary -- lying about the attack in Benghazi.

David Brock, the founder of Media Matters, has co-authored an ebook, The Benghazi Hoax, that dismantles a wide range of conservative lies about President Obama and others in the administration.

If you want the facts, you can buy The Benghazi Hoax for 99 cents at today.

David has also written an op-ed at Huffington Post that focuses on the relentless right wing campaign against Hillary Clinton. Take a moment to read the op-ed, and then consider forwarding this email to your friends and family today.

Read David Brock’s op-ed here.

It is vital that we get the truth out, and with your help, I know we will.