Kevin Counihan, the chief executive officer of the insurance exchange in Connecticut, one of the most successful in the country, said he expects one new company to join the four plans offering coverage on Access Health CT next year. About 79,000 people in the state signed up for exchange plans this year, twice as many as state officials anticipated.

‘New Culture’

“This whole state came together to try to incorporate this as a value for the state and to create a new kind of culture of assuming that this is a right of citizenship in the state of Connecticut,” he said in a phone interview.

Even if Republicans win the Senate in November congressional elections, giving them control of the entire Congress for the first time since 2006, they pose little threat to the health law with Obama still in office, said Hoagland, the former Frist adviser.

The main vulnerability in the law now comes from insurers, at least one of which -- WellPoint Inc. -- has said it may propose large premium increases next year. Insurers will file 2015 rates in May and June, and any increases are likely to be announced by the end of the summer, posing political difficulty for Democratic congressional candidates, Hoagland said.

‘Real Problems’

“It could very well be that some of those rate increases will be significant,” Hoagland said. “If you start to see double-digit premium increases, that could cause real problems for the plans, and possibly for Obamacare going forward.”

In his comments yesterday, Obama said that about 35 percent of enrollees in the federal insurance exchanges are younger than 35. That’s a four percentage-point rise since March 1, indicating a surge of sign-ups by young adults and children. That’s good news for the administration, as insurers build their rates in part by weighing the number of healthy customers they believe they’ll get compared with the number of costly older and sicker consumers.

Longer term, provisions of the law aimed at reducing health costs and improving care must be shown to work before Obamacare can be declared a success, said Doug Holtz-Eakin, the executive director of the American Action Forum, an advocacy group in Washington that is critical of Obama’s administration.

“This law is more than people being covered in exchanges and Medicaid,” he said in a telephone interview.