“By moving so aggressively and with such massive scope, it’s all his now,” said Ed Gillespie, former counselor to President George W. Bush, referring to the shift in accountability from his boss to Obama.

As Obama moves to enact his broad campaign platform, he may need to strike a balance between his core values and pragmatism, as his $3.6 trillion spending plan raises alarms about a widening budget deficit.

“You’re going to see that play itself out over and over again on all the issues we’re going to be taking on,” said former Senate Democratic leader Tom Daschle, an Obama confidant.

Before the August recess, the White House expects the House of Representatives to pass energy legislation, including a cap-and-trade system to control carbon-dioxide emissions that cause global warming, and the Senate to approve a bill overhauling the nation’s health-care system.

“Some of the major items on his agenda will be very hard to accomplish — very hard, especially cap and trade and health care,” said Bill Galston, a Brookings Institution scholar who was a domestic policy adviser to former President Bill Clinton.

Those two issues form only part of the president’s ambitious program over the next three months.

Obama, 47, will decide whether to force General Motors Corp. into bankruptcy and gauge the viability of Citigroup Inc., Bank of America Corp. and 17 other large U.S. banks. He may also learn by mid-summer whether the Treasury Department’s bank-rescue plan is working.

On foreign policy, he needs to show progress in Afghanistan and Iraq, since his proposed $83 billion war- funding measure is already drawing fire. He’ll also have to fend off Democratic calls for a truth commission to investigate alleged abuses of terrorist suspects.

In the coming weeks, Obama plans to revamp federal contracting, fight lending fraud and empower judges to write down mortgages for struggling homeowners, and he wants Congress to pass a tobacco-regulation bill. By his 200th day, he may have the chance to appoint a Supreme Court justice.

His success may require a level of combativeness — toward both Republicans and some of his own party members — that he hasn’t had to display in his first months in office.

“He may have to threaten more and promise more and throw his weight around even more,” said Stuart Rothenberg, a Washington-based independent political analyst. “He’ll have to get his hands a little dirtier.”

It’s not going to be easy, said Daschle.

“All these issues are extremely contentious, and the ideological divide is as evident as it’s ever been,” he said.

Arizona Senator Jon Kyl, the chief Republican vote- counter, said his party is ready for battle. “Will there be bipartisan cooperation to turn over our health-care and our energy policy and our education system and our financial institutions to the government? No, there won’t,” Kyl said.

The focus on the first 100 days obscured the fact that all presidencies are judged by the rest of their terms — including how they respond to unexpected crises, whether it’s pirates taking hostages off Somalia or a

potential swine flu pandemic.

“The first 100 days have been impressive, but it’s not at all as difficult as the next 1,400 or so days will be,” said James Thurber, director of the Center for Congressional and Presidential Studies at American University in Washington.

Obama’s first months in office have been consumed by the need to stabilize an economy that has been in recession since December 2007.

Now his approach is echoing that of Johnson, who believed he had to get most of what he wanted through Congress in the first six months after his 1965 inauguration, before lawmakers got distracted by re-election bids, said historian Michael Beschloss.

“That general lesson may be on Obama’s mind, so therefore we are likely to see an active second hundred days,” Beschloss said. “They presume that this is their moment.”

David Axelrod, a senior White House adviser, said that while administration officials have no interest in taking “half-measures,” they recognize “there are limits, and we know what they are.”

One example: the White House may be pushing back its goal of overhauling immigration laws. “We’re committed to beginning that discussion this year,” said Axelrod, 54. “Whether we complete that this year is another question.”

On cap-and-trade, Democrats from coal-producing states want alterations to the climate-change legislation to benefit the coal, oil and gas industries.

And some party activists and labor unions are concerned Obama won’t stick to his goal of creating a government- sponsored health-insurance plan to compete with private companies, in the face of opposition from insurers and Republicans.

Democrats are on the verge of adopting a budget-plan tactic known as reconciliation that would prevent Republicans from blocking Obama’s health plan by allowing the measure to be approved on a simple majority rather than the 60 votes needed to overcome any Republican filibuster.

Another unresolved issue is whether Obama can withstand calls to create a truth commission to explore possible human rights violations during Bush’s “global war on terror.” The issue is so contentious that even the Democratic leadership is divided, with House Speaker Nancy Pelosi among those promoting such a body and Senate Majority Leader Harry Reid opposed.

“Some of the items will almost certainly be pushed off into next year and beyond,” said Thomas Mann, a congressional scholar at the Brookings Institution in Washington. “He’s in this for eight years, not for 100 days or 200 days.”

Obama also has a full foreign policy agenda, starting with Iraq and Afghanistan and the nuclear ambitions of North Korea and Iran.

On April 21, he invited Israeli Prime Minister Benjamin Netanyahu, Palestinian Authority President Mahmoud Abbas and Egyptian President Hosni Mubarak to the White House for separate talks next month on Middle East peace. The next day, he invited Afghan President Hamid Karzai and Pakistani President Asif Ali Zardari for talks.

Senator Lamar Alexander, a Tennessee Republican, said Obama is in danger of taking on too much.

“I would have thought that fixing the banks and getting the economy back would be it,” he said. “If he makes it one of 17 issues, then it is trouble.”

That skepticism underscores why the administration needs to engage lawmakers more than it has, said Republican Ray LaHood, a former Illinois congressman now serving as Obama’s transportation secretary.

“Everyone on the team is going to have to work much more closely with Congress,” LaHood said. “It becomes a little trickier to the extent there will be a lot of players involved, not just a handful of leaders.”

Obama’s ability to get his agenda through will be bolstered by a 63 percent job-approval rating, higher than those of his recent predecessors, according to a poll released April 23 by the Pew Research Center for the People & the Press.

Added to that, stocks have climbed, with the Standard & Poor’s 500 Index rebounding 27 percent from a 12-year low on March 9, on speculation that government efforts to fix the banking system will pull the nation out of the recession.

Yet few Republican lawmakers have supported his agenda. Not one Republican House member voted for the $787 billion economic-stimulus package, while only three in the Senate did.

Obama is also likely to encounter resistance from the Blue Dog Democrats, a coalition of lawmakers concerned about the federal budget deficit.

Another potential constraint on Obama will be the state of the economy.

“The faster the economy stabilizes and begins to recover, the stronger the president will be able to push on other fronts,” said Galston of Brookings. If the economy remains as it is — or worsens — Obama’s power of persuasion will likely suffer.

Either way, the next 100 days and the rest of Obama’s term will require “a very heavy lift,” said Daschle.