By David Morgan and Roberta Rampton

WASHINGTON, Nov 5 (Reuters) - The Obama administration, under pressure over the botched opening of its healthcare website, scrambled on Tuesday to try to appease hundreds of thousands of people whose coverage is being cancelled as insurers prepare for reforms in 2014.

President Barack Obama and his top officials are trying to contain the fallout from people angry they have lost their insurance and frustrated with being unable to shop easily for alternatives on the malfunctioning website, HealthCare.gov.

Obama had repeatedly promised that under the new signature law, people with insurance would be able to keep their existing plans if they wanted to - a pledge that glossed over details of which policies would be protected from new minimum benefit requirements.

"The president, as awesomely powerful as the office is, can't go back in time," White House spokesman Jay Carney told reporters when asked whether Obama would use the same words to describe the grandfathering provision.

Obama's Chief of Staff Denis McDonough urged a group of insurance executives on Tuesday to tell consumers in cancellation notices that they could qualify for premium tax credits through the new online marketplaces.

Some cancellation victims hear only about costly replacement plans from their insurers and not about options available through the marketplaces, including the subsidies.

"He's saying that we all need to do the best we can in getting information that consumers need," Carney said.

OBAMA TO DALLAS, SEBELIUS ON THE HILL

On Wednesday, Obama will visit volunteers in Dallas who are helping people sign up for health insurance - part of a push for senior officials to highlight the program in cities with the highest number of uninsured residents.

In Dallas County, more than 670,000 people or 28 percent of the total population do not have insurance, the White House said. Texas has the nation's highest percentage of uninsured people.

Meanwhile, Health and Human Services Secretary Kathleen Sebelius will face tough questions at a Senate Finance Committee hearing on Wednesday, both from Republicans who oppose Obamacare as an unwarranted expansion of the federal government, and from Democrats dismayed at how poorly the launch has gone.

Marilyn Tavenner, the head of the federal agency responsible for the Obamacare rollout told lawmakers on Tuesday that her staff is working on a plan to get more information to people with canceled plans.

"This is actually a conversation we're having today ... Is there a way we can actively engage to reach out to people who have been canceled?" Tavenner, administrator of the Centers for Medicare and Medicaid Services, told the Senate Health Education Labor and Pensions Committee.