President Obama and his allies in Congress say the U.S. health system must change to curb surging medical costs and extend coverage for an estimated 48 million uninsured Americans.

But which of those is more important?

Though it might seem strange to choose just one goal, the staggering price tags of $1 trillion or more that have been attached to early drafts of health bills in Congress are prompting lawmakers to ponder trade-offs, and confront the unavoidable tension between cost controls and adding coverage.

At his news conference on Tuesday, Obama again emphasized the cost side of the issue, vowing to ensure that any overhaul “brings down the crushing cost of health care ... the skyrocketing costs that are driving families, businesses and our government into greater and greater debt.”

The president’s overriding focus has experts wondering how committed he is to expanding coverage — specifically, whether he will be willing to accept a politically palatable compromise that defers or stops short of the goal of providing universal care.

Health care debates have evolved in this way for decades, because most Americans believe quality care should be available to anyone who needs it, but are unwilling to finance a truly “universal” system out of taxpayer dollars. These seemingly contradictory stances have prompted policymakers to jury-rig the system by shifting costs, subsidizing poorer patients’ care and paying providers based on predetermined costs.

Administration officials clearly have a political incentive to play up the dollars-and-cents aspects in the current debate, in order to firm up their bona fides as responsible stewards of the economy and deprive Republicans of a potential campaign issue in 2010 and beyond.

Obama and his aides are battling intensifying perceptions that the administration is enlarging deficits through its ambitious social programs and is incapable of controlling spending.

“If the Republicans have anything going for them, in two years they might be able to say, ‘We told you these guys are tax-and-spend liberals,’ ” said Rick Weissenstein, a health care analyst with the Washington Research Group. “I think the White House believes they can bend the growth curve [on medical inflation.] This is definitely becoming as much about savings as about the uninsured.”

It is for this reason that White House officials identified billions of dollars of efficiencies that could be squeezed out of the health system even before draft plans were unveiled in the House and Senate. Obama and his aides have also spoken at length about a next-generation health system that places value over volume, holds doctors more accountable for results and pays on the basis of performance.

The White House approach is surely influenced by the experiences of the Clinton administration, which based much of the justification for its failed health care plan in 1993 and 1994 on providing permanent health coverage to all Americans. Hillary Rodham Clinton , who as first lady was heavily involved in selling that plan, chose to make universal coverage a major theme of her presidential campaign as well.

Obama, in contrast, offered a health plan that contained no firm mandates for universal coverage, arguing they could penalize people with modest incomes.

By deferring to Congress to define the contours of this year’s debate, experts say Obama has left himself a lot of wiggle room to find the savings he’s seeking — and still portray any resulting plan as true to his principles.