HarborOne Bancorp Inc. has filed plans to go public and sell shares in a partial stock offering, a move that bank executives say is necessary to raise capital, while critics argue that it means a big payday for insiders.

Once the state’s second-largest credit union, HarborOne converted into a cooperative bank in 2013. Now, it plans to sell just under 50 percent of shares in the bank to create a mutual holding company and raise as much as $136 million to expand its franchise, fund new loans, and improve existing products.

The conversion from a credit union three years ago was controversial, viewed by critics as an incremental step toward a public stock offering that would benefit management at the expense of consumers.

Interest rates on car and home loans tend to be lower at credit unions, since there are no stockholders who demand dividends, said Debbie Matz, chairwoman of the National Credit Union Administration, the chief regulator for credit unions.

Matz predicted that HarborOne’s conversion to a cooperative bank would probably lead to public stock sale. She said Monday that she was “disappointed,’’ but not surprised by HarborOne’s move.

“My first reaction was ka-ching, ka-ching, ka-ching,’’ Matz said. “The only reason to convert is to enrich the board members and management.’’

James W. Blake, HarborOne’s chief executive, disputed Matz’s assertion. “We’re doing this to grow,’’ he said.

Since HarborOne abandoned its credit union structure, Blake’s compensation has increased nearly 60 percent, to $1.2 million last year from about $727,637 in 2013. Much of that increase was based on performance bonuses.

The bank’s board of directors, once essentially unpaid positions, earned between $18,000, for a half year of work, to nearly $63,000 for all of 2015, in fees, according to HarborOne’s filings.

About one-fifth of the shares sold in the IPO would be set aside for bank officers and employees to purchase or receive as grants.

Executives and other insiders would have an opportunity for another payday by selling to larger institution willing to pay a premium price.

For example, Danversbank in Danvers became a publicly traded banking company in 2008 and then sold itself to People’s United Financial Inc. in Connecticut three years later. The Danversbank chief executive received $16 million in compensation and other payments related to the sale.

Blake said HarborOne has no plans to sell the institution and that this is only a partial public offering. The bank would have to sell the majority of its stock in another sale before it could be acquired by a stock bank.

“The fact that we’re doing a minority offering is an absolute signal to Wall Street that we have no intention to cash out,’’ Blake said. “I have no intention of cashing out.’’

Blake, however, didn’t discount that HarborOne may need to sell more shares in the future, but added such a sale would be driven by the need for additional capital, he said.

In the past year, HarborOne has bought a mortgage company and expanded its commercial lending business into Providence. To finance additional growth, HarborOne needs to look to outside investors, Blake said.

The bank has grown to $2.2 billion in assets from less than $2 billion at the end of 2013 now. It grew its commercial and industrial loan portfolio to $57 million last year from $14.8 million in 2013.

“This makes us competitive,’’ Blake said of the public offering.

HarborOne depositors will vote to approve the conversion on March 29th. State and federal regulators will also have to sign off on the public offering.

Blake said he expects Harbor­One to have the proceeds from the sale of stock by July 1.

Deirdre Fernandes can be reached at deirdre.fernandes-@globe.com. Follow her on Twitter @fernandesglobe.