WASHINGTON — The White House, backing away from the “it’s-all-on-the-table” approach that President Barack Obama initially advocated, prepared to get louder and more involved in the details of a health care overhaul that officials once were content to leave to Congress, administration officials said Saturday.

The White House’s attention increases as Congress turns to a priority that officials watched in recent weeks drift off what has otherwise been a precise pathway. Even with an Obama-imposed August deadline, many administration aides weren’t sure just how much they would be able to accomplish before Congress left for the summer, and Obama has turned to his grassroots supporters to pressure Congress to find a solution.

Covering 50 million uninsured Americans could cost as much as $1.5 trillion over a decade, but Obama has cited the crippling impact on the economy of soaring health care costs and society’s long-standing need to resolve the problem. Obama and lawmakers say they want to lower costs, ensure choice and provide coverage to those who are uninsured. Obama and his advisers initially let Congress take the lead, remembering what happened when President Bill Clinton took to Congress a plan deemed too detailed and too prescriptive.

Draft legislation from the Senate Health, Education, Labor and Pensions Committee would require employers to cover their employees or pay a penalty and would guarantee coverage for all.

But Obama supports a new public insurance plan that would give all Americans the opportunity of getting government-sponsored care. Private insurers are adamantly opposed, fearing they’d be driven out of business, as are most Republicans. And Obama’s team has grown more willing to take the lead.

“If we do nothing, everyone’s health care will be put in jeopardy,” said the president, in his Saturday radio and Internet address. “Fixing what’s wrong with our health care system is no longer a luxury we hope to achieve — it’s a necessity we cannot postpone any longer.”

Congress still hasn’t figured out how to pay for the health care overhaul. Obama has put forward some ideas, including cuts to Medicare and Medicaid. Others he’s suggested, including limiting some high-income tax deductions, have already been shot down on Capitol Hill.

Obama has stated a preference for a bipartisan solution, but that’s looking harder to achieve.

A long-planned grass-roots effort is meant to illustrate power and to intimidate opposition. It is coming with Obama’s explicit blessing, according to officials.

Aides at Organizing for America — as Obama’s political arm is known — said tens of thousands of supporters participated in thousands of events for health care overhaul on Saturday.

With some 14 million e-mail addresses and an Internet-based advocacy machine that helped him win an election, Obama’s political arm sought to deliver changes to the health system similar to the ones Obama talked about during the campaign — not one that was mauled through endless compromise or one far different from the one that motivated thousands of volunteers last fall.

Obama has indicated he wanted to hear Democrats’ and Republicans’ ideas, but has also told them that he’s the president and they are among many elected members of Congress. They also were warned that the re-election campaign-in-waiting was revving its engines.

That part of the message was clear, even if other pieces were not.

After Senate Democrats met with Obama and his top aides in the State Dining Room, one of the president’s fellow Democrats marched to a microphone and declared that the administration was open to taxing health care benefits — something Obama opposed during campaign and remains personally against.

“It’s on the table. It’s an option,” said Senate Finance Committee Chairman Max Baucus, D-Mont. The problem, Obama aides later reluctantly acknowledged: Obama had said exactly what the powerful chairman said.

Obama had refused to declare any provision “a sacred cow,” according to an official familiar with the negotiations. Although Obama’s personal position on the tax is clear, he had sought to let lawmakers fashion the exact language.

No more, Obama’s team said. If the president wants health care overhaul, the White House needs to control what’s being said, not its allies — or, worse, its rivals.

“This issue, health care reform, is not a luxury,” Obama said in his address. “It’s not something that I want to do because of campaign promises or politics. This is a necessity. This is something that has to be done.”

Obama summed up his message with a simple declaration: “It’s time to deliver.”