WASHINGTON (Reuters) - President Barack Obama on Tuesday called for reforming the gigantic government pension and healthcare programs, as well as simplifying the tax code -- steps that would go far beyond the $3.7 trillion budget he proposed this week.

At a White House news conference, Obama tried to quell Republican criticisms of his fiscal policies and called on the opposition party to join him in reducing the costs of the major entitlement programs of Social Security, Medicare and Medicaid.

He said he wanted to simplify the unwieldy tax code, which also could help reduce a U.S. budget deficit that the White House estimates will hit $1.65 trillion this year.

"All of these steps are going to be difficult. That's why all of them will require Democrats, independents and Republican," Obama said, adding, "everyone is going to have to give a little bit."

But Republicans in Congress were prepared to launch what promises to be a raucous debate over government spending in the House of Representatives.

They hope to pass by week's end legislation to cut current spending by at least 14 percent, or $61 billion. Conservatives -- with Tea Party backing -- were hoping to slice much deeper with hundreds of amendments that they hope to test on the House floor.

Obama's budget did not touch the biggest drivers of the deficit, mandatory spending for Social Security pensions, Medicare and Medicaid healthcare programs for the poor and elderly, which make up about two-thirds of the budget.

White House Budget Director Jack Lew traveled to Capitol Hill to defend Obama's budget blueprint for next year, only to hear Republicans accuse the president of failing to tackle the nation's deep fiscal problems.

"The president's budget disregards the drivers of our debt crisis and the insolvency of our entitlement programs, House Budget Committee Chairman Paul Ryan told Lew.

Republicans on the tax-writing House Ways and Means Committee are expected to give a similarly rough ride to Treasury Secretary Timothy Geithner when he testifies later on Tuesday.

Meanwhile, Commodity Futures Trading Commission Chairman Gary Gensler and Securities and Exchange Commission head Mary Shapiro told Congress the Republican spending cuts would hurt both agencies' enforcement activities.

Democrats say the House Republicans' spending cuts will hurt vital government programs and possibly threaten the economic recovery.

(Editing by Jackie Frank)