WASHINGTON -- President Obama told his House Democratic allies Monday that despite a weak economy, now is the time to take on ambitious agenda items such as education, health care and clean energy -- or else they won't get done.

Obama's closed-door appearance comes as the House and Senate this week take up companion budget outlines that constitute their initial response to the $3.6 trillion fiscal plan for 2010 submitted by Obama last month.

Typically, the first year of a presidency -- when a president has political capital from a winning election -- is the most opportune time to take on big issues.

Obama assured the assembled lawmakers that the country has confidence in Democratic leadership and that continued unity is needed to produce results that will in turn keep the party's standing high with the public, according to notes taken by a House aide who required anonymity to describe the private session.

"We are in this together," Obama said.

Speaking to fiscal hawks in the House, Obama said that the reason his budget predicts such large deficits is not because of new spending for health care, energy and education but because of structural deficits that he inherited.

Obama said he is "serious as a heart attack" about addressing the nation's long-term deficit problems.

Rep. Chris Van Hollen, D-Md., said Obama said his administration remains focused primarily on the economy but added that "'we can do more than one thing at a time."'

Obama appeared before Senate Democrats last week, where he received supportive questioning as moderates queasy over big deficits and rapidly rising debt held their tongues.

Meanwhile, debate started in the Senate on that chamber's response to the budget Obama submitted in February.

Obama's Capitol Hill allies have kept their budget blueprints close to Obama's despite being forced by worsening deficit projections to pare back some initiatives.

In the absence of key details on signature Obama proposals such as how to fulfill his promise to guarantee health care coverage or reduce U.S. emissions of heat-trapping gases, the nonbinding congressional budget plans track Obama's plans only generally in key respects.

Obama, according to notes taken by a second aide, said a vote for the budget is a necessary first step toward implementing his ambitious but politically difficult agenda.

"We will create a sense of momentum that will allow us to do health care reform and education" and other major initiatives, Obama said.

Debate on the Senate's budget plan opened Monday, with a vote expected Thursday night or Friday. On Thursday, the House is expected to approve its plan, which promises slightly higher spending and somewhat worse deficits.

House Speaker Nancy Pelosi, D-Calif., promised: "Yes we will pass a budget this week."

Under Congress' arcane budget legislative process, lawmakers devise a nonbinding budget resolution that sets the terms for subsequent legislation. As a practical matter, the most immediate impact is to provide a pot of money to the appropriations panels to fund Cabinet agencies' annual budgets.