"What you need to do is continue what you're doing and reach out with your teams in your respective cities, states, towns, counties because right now we only have a few weeks left," he told an organizing summit for Organizing for Action. "March 31st, that's the last call."
The White House has set an unofficial goal of 7 million enrollees by the end of March.
"If they want health insurance now, they need to sign up now, and we're going to make a big push these last few weeks," Obama said.
Nearly 3.3 million had enrolled through the end of January. Enrollment was slowed in the beginning by the rocky start to the administration's healthcare website.
"Let's face it, the website didn't work," Obama said.
Obama also attributed enrollment troubles to an "implacable opposition" that has spent hundreds of millions of dollars if not billions to oppose the healthcare law.
In a statement, Marilyn Tavenner, the administrator of the Centers for Medicare and Medicaid Services, said the administration is seeking "strong demand nationwide from consumers who want access to quality, affordable coverage." She said system error rates are low and "response times are consistently less than half a second."
In addition to the 4 million enrolled, Obama said millions more were benefiting from the healthcare law's expansion of Medicaid and its provision allowing young people up to age 26 to remain covered by their parents' insurance.
Signing up enough individuals — especially younger, healthier people — is critical for the insurance pool at the heart of the law to function properly by keeping premiums low for everyone.
Obama spoke to more than 300 activists at OFA's organizing summit at a Washington hotel and later addressed about 80 supporters at a dinner. The group heard from Jim Messina, Obama's 2012 campaign manager, and several former Obama aides who encouraged attendees to help enroll people under the healthcare law.
"The last call is here and it is our job to let everybody know," said Jon Carson, OFA's executive director.
Two hours before the president spoke, his former Republican rival, Mitt Romney, was spotted in the hotel lobby. A Romney adviser said the former Massachusetts governor was in Washington to deliver a speech and was staying at the hotel by coincidence.