Former Secretary of State Hillary Clinton meets and speaks to Iowa voters at Grand View University in Des Moines on Friday. (Photo by Melina Mara/The Washington Post)

Priorities USA Action, one of two super PACs backing Democratic presidential contender Hillary Clinton, has raised $50.5 million to date and is ending the month with nearly $45 million in the bank, giving it ample ammunition to bolster Clinton as she tries to fend off a strong challenge from Sen. Bernie Sanders.

In a memo, chief strategist Guy Cecil wrote that donors have committed to give another $42 million to Priorities. With that, the group is on track to bring in $92.5 million, nearly $14 million more than the super PAC raised in the entire 2012 cycle, when it was supporting President Obama. The group's fundraising totals were first reported by The New York Times.

Major backers this cycle include some of country's largest labor unions, including the American Federation of Teachers, a longtime supporter of Bill and Hillary Clinton's political and philanthropic endeavors.

The super PAC has not launched any attacks against Sanders to date. So far, it has reported spending just $440,000 on pro-Clinton ads, casting them as an effort to defend her against Republican assaults.

"They are preparing a billion-dollar campaign to deceive and distort her record for months on end," Cecil wrote. "Priorities USA sees it as our responsibility to fight back early and often."

But the spots also serve to boost her credentials among Democratic primary voters as she tussles with Sanders for their support. The newest Priorites ad features Peter Read, whose daughter Mary Read was killed during the 2007 Virginia Tech massacre. In it, he praises Clinton for taking on gun right groups, an issue Clinton has seized upon as an opening against Sanders.

After news of Priorities’ fundraising haul broke Friday, Sanders’ campaign released a press release saying he raised no money for a super PAC last year.

“Bernie doesn’t want billionaires’ money. He doesn’t have a super PAC. He believes you can’t fix a rigged economy by taking part in the corrupt campaign finance system in which politicians take unlimited sums of money from Wall Street and other powerful special interests and then pretend it doesn’t influence them,” spokesman Michael Briggs said in a statement.

However, Sanders is being supported by a super PAC financed by National Nurses United, which has so far spent more than $1 million touting his candidacy.

Anu Narayanswamy contributed to this report.