Former New York Mayor Michael Bloomberg arrives to speak with Virginia House Delegate-elect Nancy Guy after launching his presidential campaign in the D'Egg cafe in Norfolk, Virginia, November 25, 2019.

A gun safety group co-founded by presidential candidate Mike Bloomberg raised its most money ever in 2018, the year Democrats took back the House of Representatives — with much of it coming from the billionaire former New York mayor himself.

The organization, Everytown for Gun Safety Action Fund, brought in $69.5 million in total revenue last year, according to a new 990 tax return first reviewed by CNBC. That's nearly double its revenue from 2017. The National Rifle Association, the biggest pro-gun lobby in the country, raised $352 million last year, meanwhile, representing only 12% revenue growth, its most recent disclosure form shows.

The 501(c)(4), which focuses on political initiatives, received $66 million of that total in grants and contributions. A source said $38 million of that — listed as an anonymous donation in the document — came from Bloomberg himself. Combined with their other nonprofit, Everytown for Gun Safety Support Fund, the organization as a whole raised over $106 million.

The source said Bloomberg, who earlier this week launched his bid for the 2020 Democratic presidential nomination, has accounted for about a third of the group's total funding in recent years. Bloomberg has a net worth of just over $55 billion.

A spokesman for Everytown noted that between the two nonprofits, they received over 300,000 online donations, which are not listed on their disclosure forms.

The organization said it spent $2.5 million this year in the most recent round of Virginia state elections, helping Democrats retake the legislature for the first time since 1994 — and outpacing the NRA, which spent approximately $300,000.

In 2018, Everytown's Action Fund made political contributions of over $17 million and spent $4 million on lobbying. Democrats made inroads that year in state governments and also went on to capture the House. Since then, House Democrats have opened an impeachment investigation into President Donald Trump.

The group highlights its advocacy on the state level, which it says has led to massive changes on gun policy.

"The greater accomplishments came at the state level, where the action fund's advocacy led to the passage of meaningful gun safety legislation in 20 states," the 990 filing says.

The disclosure of Everytown's success last year comes as Bloomberg runs for president with a message that he is the only candidate who can defeat the gun lobby.

The group's other affiliated nonprofit, Everytown for Gun Safety Support Fund, which is a 501(c)(3) dedicated to gun safety education, raised $37 million last year.

Data compiled by ProPublica, which tracks the activity of nonprofits, shows the closest Everytown has come to its 2018 revenue total was in 2016, when it raised $52 million. The group was formed in 2013.

Trump has been a major beneficiary of the NRA, and the group remains the most influential advocate for gun manufacturers. Trump has repeatedly said he would push for background checks and other gun reforms, but has backed off those ideas after consulting with NRA officials, particularly the group's chief, Wayne LaPierre.

Despite raising more than $350 million last year, however, the NRA's nonprofit, along with its four affiliated charities and its political committee, ended the year $10.8 million in the red, The Washington Post reported.

Bloomberg has made taking on the NRA a key part of his presidential campaign, which he launched Sunday.

"I'll be making gun violence a major issue of my campaign, just as I made it a major issue of my life in government and philanthropy," Bloomberg told reporters Monday in Virginia. "I spent hundreds of millions of dollars fighting the NRA to win common-sense gun laws, and I'm not stopping now."