NEW YORK — Text messaging is posing both new opportunities and dangers for America's political campaigns.

The most widely used form of mobile communication, it has become one of the most effective ways for campaigns to reach supporters, using 160-character messages to encourage last-minute donations or provide information such as where to vote. And strict federal rules prohibit such texts from going to anyone who does not "opt in" to receive them.

But some groups have found their way around that requirement, using email — rather than the SMS "short code" that telemarketers normally use — to send unsolicited, anonymous and often negative messages to cellphone lists they purchase through brokers.

That texting practice has angered voters, who are forced to pay if they don't have flat-rate messaging plans. And it's alarmed campaign strategists, who fear political texting will be weakened by the introduction of what amounts to spam texting.

"They've taken a tool and technology we used to help people get voter information and turned it into a very sophisticated way to do voter suppression tactics and annoy people with false and misleading information," said Scott Goodstein of Revolution Messaging, a Democratic-leaning mobile communications firm. "Worse yet, people are being charged to receive these messages."

Goodstein has filed a complaint about the practice with the Federal Communications Commission, whose Telephone Consumer Protection Act prohibits telemarketers from texting "to any telephone number ... or any service for which the called party is charged."

Unsolicited messages hit the presidential campaign this year, when texts targeting Republican Mitt Romney surfaced in Colorado, South Carolina and Michigan. Voters received texts urging them to call a number where they heard a recorded message criticizing the former Massachusetts governor.