In the great debate over the federal budget, liberal Democrats jousting with the Reagan administration are fighting for propositions that would probably make Hubert H. Humphrey turn a somersault in his grave. But that's how politics have changed in the Reagan era.

The battle in the Democratic-controlled House will be fierce in the days and weeks to come as old-line Democrats struggle to save some of their favorite programs and preserve their approach to governing.

The White House is extremely anxious to push its version of the 1982 budget through the House to discredit that Democratic approach and to demonstrate the new president's political power.

So the fight will be both symbolically and substantively important. But that fact should not obscure another basis reality: Ronald Reagan has already succeeded in pushing the middle of the American political debate far to the right in his 10 weeks as president.

It is a tribute to Reagan's success that House Speaker Thomas P. (Tip) O'neill Jr. (D-Mass.) has embraced an alternative budget proposed by Rep. James R. Jones (D-Okla.) that would leave intact huge cuts in many of the programs O'Neill and his colleague once trumpeted with pride.

Through unemployment is still 7.3 percent, for example, the O'Neill-Jones budget would eliminate the Comprehensive Education and Training Act (CETA) public service jobs program.

The Democrats have chosen to fight for principles, not for dollars. As an aide to O'Neill put it yesterday, the Jones budget is an attempt to force Reagan to live up to his rhetoric by retaining historically Democratic means to help needy Americans.

"He says he's concerned about the truly needy, and we say his budget doesn't reflect that fact," this Democrat said.

Jones' budget suggests restoring about one-third of the money Reagan wants to cut from the food stamp program. Where Reagan proposes cutting $1.1 billion in 1982 from aid to elementary and secondary education, Jones suggests cutting $600 million. Jones asks for a revival of the Legal Services Corp., which Reagan would abolish, but with a budget two-thirds of its current one.

The only significant case where Jones has sought to revive thoroughly a big social welfare program targeted by Reagan is Medicaid. The administration proposed a cap on the total cost of the program, which provides health care to the poorest Americans. Jones' budget would remove the lid, a change that would cost $1.6 billion.

Despite Jones' generally modest attempts to bring back favorite Democratic programs, his efforts have been warmly embraced not only by O'Neill but by staunch young liberals in the House.

For example, Rep. Thomas J. Downey (D-N.Y.), a leader of liberal efforts to deny Jones the chairmanship of the House Budget Committee in January, said this week that Jones' budget is a splendid accomplishment which Downey endorsed unreservedly. Other liberals spoke in a similar vein.

Embracing Jones' budget means embracing the biggest single-year increase in the defense budget in American history, not a cause long associated with liberal Democrats. Though Jones proposes modest cuts in Reagan's defense-spending proposals, the savings would come largely from skipping a military pay increase the administration favors.

Jones made no attempt to eliminate any of the billions Reagan has added to former president Carter's already-large defense budget for new procurement of military hardware. Not even the liberals on the Budget Committee proposed reducing procurements. "It's no use," one of them said. "We don't have the votes."

House Democrats are hoping they do have the votes to save a Democratic approach to some of society's problems in this Republican year. And if the Jones budget were to win eventual approval, it would tangibly affect some important Democratic constituencies.

For instance, Mayor Edward Koch of New York City estimates that the president's budget would cost New York City $350 million, but Jones' budget would take less than half that much from his city's coffers.

In the end, though, neither the Republican-supported budget resolution passed by the Senator nor whatever budget resolution is passed in the House is likely to survive the rest of this year.

The heart of the budget process is in the appropriations committees, which decide how to divide up the federal pie. They'll do their work in the summer and fall.