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New York Mayor Michael Bloomberg is losing part of his arsenal of local leaders as more and more long-time members of his gun-control group Mayors Against Illegal Guns say they aren’t happy with the coalition’s trajectory and want out.

While the group apparently is growing in membership overall amid an effort to assume a larger profile in the national gun debate, it's turning some members off. In the past five months, 50 members of the group have quit. Many say they did so because the organization abandoned its mission statement of going after illegal guns, and instead used its political clout to go after lawmakers who supported gun rights.

Most recently, the mayors of Rockford, Ill., and Nashua, N.H., dropped out after saying they felt misled by Bloomberg.

Nashua Mayor Donnalee Lozeau says she called it quits after the group launched television attack ads against New Hampshire Sen. Kelly Ayotte. Ayotte voted against legislation that would have expanded background checks to cover almost every gun purchase in the country.

“I said, ‘Wait a minute. I don’t want to be part of something like that,’” Lozeau told The Manchester Union Leader. “I told them, ‘You’re Mayors Against Illegal Guns; you’re not mayors for gun control.’”

While the hits to membership have stung for Bloomberg, the organization is still the country’s leading gun control group and offers a counter-voice to the National Rifle Association. MAIG was co-founded in 2006 by Bloomberg and Boston Mayor Thomas Menino. According to its web site, the group started off with 15 members but has ballooned to 950 mayors from 45 states, representing Republicans, Democrats and Independents from large cities and small towns across the country.

That's still a fraction, though, of the total number of mayors and local executives in the country -- representing just 5 percent of the leaders of America's 19,000 municipalities.

Rockford’s three-term independent Mayor Lawrence Morrissey also recently bowed out of Bloomberg's group.

“The reason why I joined the group in the first place is because I took the name for what it said – against ‘illegal’ guns,” Morrissey said at a town hall meeting on June 22. His comments were met with applause. “I thought it was about enforcement of (the) existing gun laws” against illegal weapons. “As the original mission swayed, that’s when I decided that it was no longer in line with my beliefs.”

Sen. Charles Schumer, D-N.Y., also questioned MAIG’s motives and political strategy after the group spent money targeting vulnerable Senate Democrats in conservative states who didn’t back the bill to expand background checks.

“The mayor of New York City putting ads against people in red states is not going to be effective,” Schumer said.

Despite reports that mayors are bolting, Bloomberg spokesman John McCarthy recently told the Sioux City Journal the group is still growing.

"Membership has actually increased by more than 50 percent from one year ago," he said. "We now have more than 1,000 mayors in the coalition nationwide, compared to 650 last year."

Sioux City Mayor Bob Scott left the organization in February. He had joined after taking office last year. McCarthy said 171 mayors have signed up since Scott's departure.

MAIG gained national prominence after the Dec. 14 elementary school shooting in Newtown, Conn., that left 20 first graders and six teachers dead.

Bloomberg, who had a net worth of $27 billion as of March 2013, has pumped millions of his own dollars into the organization, which in recent months has been dealing with a spate of controversies.

Last month, the group had to apologize after the name of Boston bombing suspect Tamerlan Tsarnaev was read aloud along with the names of gun violence victims, at a MAIG-organized event in New Hampshire.

MAIG also recently got into some hot water following allegations it was using city web servers to host MAIG’s website. That’s a problem because the organization is a registered 501(c)(4) non-profit, which is supposed to be separate from the city government. That also happens to be the class of non-profit that many Tea Party and conservative groups were delayed or blocked from forming due to heavy IRS scrutiny.

Questions about whether Bloomberg was using city servers and workers for MAIG-related work were first raised by blogger John Ekdahl on June 21.

“It turns out I didn’t even have to look that closely, because the mayorsagainstillegalguns.org domain is registered to the city of New York,” Ekdahl wrote. More specifically, its domain is registered to the New York City Department of Information Technology & Telecommunications.

“At best, this is really sloppy,” Ekdahl wrote. “At worst, this could be pretty serious.”

Requests from FoxNews.com for comment from City Hall as well as from MAIG were not returned.