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. …

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.