Premiums varied widely in many markets during the last enrollment period, which ended in March. But as a handful of states have released insurers’ proposed premiums for 2015, the difference appears to be narrowing, said Brett Graham, a senior executive for Leavitt Partners, a consulting firm, and he has not seen any insurers leave the market.

Early filings in Washington and Virginia show potential rate increases in the single digits.

WellPoint, which has a large presence on the state exchanges, says it has no plans to leave any of its markets, and Aetna has also indicated it will most likely be in about the same number of markets. Humana says it has not yet decided what it will do.

Several smaller insurers say there are opportunities for expansion, especially in those states where competition is limited. In New Hampshire, for example, Harvard Pilgrim offers employer coverage in the state and had already planned to begin selling individual policies through the state exchange.

“It’s not really a place where very large-scale carriers are going to make a decision to come in,” said Beth Roberts, a senior vice president for the insurer. “It’s a perfect opportunity for growth for us.”

Harvard Pilgrim says it waited a year because of the logistical hurdles involved in offering coverage in the state, not for any lack on interest on its part. “It was really an issue of not having the ability to ramp up as quickly as we needed,” Ms. Roberts said.

The other insurer planning to join the exchange, Minuteman Health, is one of the 23 nonprofit cooperatives set up under the law to increase competition. In Massachusetts, Minuteman distinguished itself through its low prices, the result of a narrow network of hospitals and doctors. In New Hampshire, the co-op said it hoped to win customers by focusing on those individuals who do not yet have insurance. “The game there is to try to knit together the best access while having a competitive price point,” said Thomas D. Policelli, the co-op’s chief executive.

Idaho has a similar dynamic, where the Blue Cross plan has long been dominant, according to Jerry Dworak, the chief executive of the Montana Health Co-op, which managed to seize 40 percent of the exchange market in that state. Blue Cross of Idaho, where Mr. Dworak once worked, captured the bulk of the exchange business. “We came to the conclusion Idaho really needed a co-op to compete with the Blues,” he said.