White House officials, fearful that the federal health care website may again be overwhelmed this weekend, have urged their allies to hold back enrollment efforts so the insurance marketplace does not collapse under a crush of new users.
At the same time, administration officials said Tuesday that they had decided not to inaugurate a big health care marketing campaign planned for December out of concern that it might drive too many people to the still-fragile HealthCare.gov.
With a self-imposed deadline for repairs to the website approaching on Saturday, the administration is trying to strike a delicate balance. It is encouraging people to go or return to the website but does not want to create too much demand. It boasts that the website is vastly improved, but does not want to raise expectations that it will work for everyone.
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"We are definitely on track to have a significantly different user experience by the end of this month," Kathleen Sebelius, the secretary of health and human services, said Tuesday. "That was our commitment."
Ms. Sebelius, who supervised development of the troubled website, tried to rally state and local elected officials in a conference call organized by the White House. "I would urge you and your folks on the ground to not hesitate to recommend that people go to HealthCare.gov and get signed up," she said.
Officials said the website was now able to handle 50,000 users at a time, providing enough capacity on a daily basis to enroll millions of people in the next four months.
But those charged with fixing the site worry that 250,000 people might try to use the site simultaneously at times on Saturday and in the days ahead. They say that pent-up demand for insurance in the federal marketplace, combined with a surge of interest among people merely curious about whether it is working, could bring the website to a crawl.
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"Our concern is that we want to make sure that people have the right expectation going into this," said Jennifer Palmieri, the White House communications director. "Early October was a frustrating experience for users. We are preparing for the outcome that we have as many or more visitors as we had on Oct. 1."
About 4.7 million people visited the website on that first day, the administration said.
White House officials offered a similar message on Monday in a meeting with some of their allies, including the Service Employees International Union and Enroll America. Both groups had pledged to work hard to drive traffic to the website.
In the meeting, Ms. Palmieri said, officials urged the groups to hold back, at least for the first several days of December, to see how much traffic the website is getting. Ms. Palmieri summarized the message: "Our recommendation is that we expect there to be really high traffic. You shouldn't be driving traffic."
If the website again crashes repeatedly, Mr. Obama's critics are sure to pounce and some of his Democratic allies may become anxious and apprehensive. Since the website went live eight weeks ago, the botched system has become a symbol of what Republicans say is an overreaching, incompetent federal government. The debate about the website's failures, and the broader role of government, may shape the outcome of next year's congressional elections.
Aides to Mr. Obama have spent the last several days trying to manage expectations by offering high estimates of potential traffic at the end of the month, and by acknowledging that the system will still have problems in the weeks ahead.
"The system will not work perfectly," Jeffrey D. Zients, who took over management of the website repair effort, told reporters on Friday. He said the site would be able to handle 50,000 users, but added, "To be clear, there will be times that volume on HealthCare.gov will exceed this capacity."
Luke Chung, the president of FMS, a database company in Virginia, said building the website to handle 50,000 simultaneous users was "not unreasonable." But he said the government must be prepared to handle much larger numbers at peak times like Dec. 23, just as the Internal Revenue Services does at the tax filing deadline in April.
In an effort to ease pressure on the website, officials have created what they call a waiting room for times when the site is operating slowly. People can ask the government to notify them by email of a better time to use the site, and they will then go to the front of the line, officials said.
Earlier this fall, the administration had envisioned an all-out push to persuade millions of uninsured Americans to log on to HealthCare.gov to buy insurance.
Instead, officials said the government would focus its efforts in December on people who had lost insurance because of cancellation notices and people who started but did not complete applications on the website.
Officials said they would also focus on urban areas in Texas, Florida, California, Illinois and a few other states with high concentrations of the uninsured. Ten counties have 18 percent of the nation's uninsured, and more than half of the uninsured live in just 4 percent of all counties, according to data from the Census Bureau and Enroll America, a nonprofit group trying to expand coverage.
The White House, pummeled daily by critics of the federal website, is stepping up efforts to persuade states to expand Medicaid for lower-income people. "Nearly half of states are so locked into the politics of Obamacare that they're willing to leave nearly 5.4 million of their own people uninsured," the White House website says.
The White House is even trying to win support in Kansas, where a Republican governor and Republican legislators have resisted expansion, citing concerns about the eventual cost. "There is no justification for continuing to block Medicaid expansion," Josh Earnest, a White House spokesman, said last week in a conference call for Kansas journalists.
But Representative Tim Huelskamp, Republican of Kansas, said: "We don't need the likes of Kathleen Sebelius or Barack Obama telling us what we should like or need. Obamacare is on life support, and President Obama keeps giving it another jolt to keep it alive."
Consumers must sign up by Dec. 23 to obtain coverage that takes effect on Jan. 1. The open enrollment period runs through March 31 for coverage that starts later. Federal officials expect three-fourths of the enrollees to qualify for subsidies, though it appears that no more than one-third qualified in the first month.