For the 50,000 or so municipalities and agencies that issue roughly $350 billion in bonds annually, the current muni market offers many benefits. Capital costs are typically low for issuers. Because the bonds are usually exempt from federal and state taxes, and sometimes local taxes, investors are willing to accept lower yields than for taxable bonds. And the fees that issuers pay Wall Street bankers have dropped steeply since 2008, when many municipalities took big losses because of ill-advised derivative deals.

Municipal governments in the bond market face challenges, though. Some banks have exited the market, put off by the thin margins that come with originating muni bonds. Structural and regulatory changes have also made the bonds less attractive to banks.

Yet as America’s infrastructure ages, the need for state and local spending on road, bridge and building repairs is mounting. As a result, some states and cities have begun experimenting with new ways of pulling in capital, including green bonds for environmental projects; mini-bonds in lower denominations that ordinary investors can more easily afford; and more direct sales to individual investors.

“We want to create a more intimate link between issuers and investors — not just the large investor,” said Steven Grossman, former state treasurer of Massachusetts, which sold the country’s first green bonds and started an online ordering system that gives individual investors direct access to new bonds.

Muni bonds are historically a safe asset class, but some are looking more risky lately as many cities struggle with underfunded pension obligations.

“You want to make sure that people understand the risks that they are getting into,” said Tom Kozlik, a municipal credit analyst at Janney Montgomery Scott. “There’s a lot of stuff you need to be paying attention to, especially in this market.”

Mr. Kozlik does not see a big need for crowdfunding in municipal finance. For one thing, he said, the market is pretty efficient as it is. He also worries that municipal bonds could be difficult for unsophisticated investors to assess. In addition to underfunded city pension obligations, he points to high-yielding bonds for charter schools as an area of risk.