Sen. Dennis DeConcini (D-Ariz.) yesterday promised to deliver what may well be the crucial vote for President Clinton's compromise economic plan. But it's not over yet.

DeConcini, one of six Democrats who voted against the package the first time it was considered, won some major concessions from Clinton before announcing his support. "I've concluded . . . the American public is tired of gridlock, they're tired of inaction by Congress, they're looking for change, and President Clinton has offered that change," said DeConcini, who weighed the risks of backing a major tax increase and spending cut bill while facing a tough reelection campaign.

But while Clinton and congressional Democratic leaders voiced growing confidence yesterday that Congress would approve the compromise budget, which would cut the projected federal deficit by $496 billion over five years, some senators were still publicly undecided. The House is tentatively scheduled to vote on the budget today, with Senate action scheduled for Friday.

"I'd hate to characterize it as a done deal. It's not as if we can crack a beer and go home," said White House press secretary Dee Dee Myers.

The Clinton administration worked at a feverish pitch to sell the package, making last-minute deals wherever needed. To counter Republican charges that it is unfair to make the income tax increases called for in the legislation retroactive to Jan. 1, Congress and the administration agreed yesterday to allow the higher-income taxpayers who must pay more to pay the increased taxes over two years without penalty.

But the day's most important announcement came from DeConcini, who would have likely doomed the plan if he voted no. Still, the drama is not over. Sen. Bob Kerrey (D-Neb.), a rival of Clinton's during the presidential campaign, is keeping Clinton on tenterhooks with repeated hints that he might switch and vote against the plan on final action.

"I don't think the Senate will let this plan go down," Clinton said after a morning meeting and pep rally on Capitol Hill with House Democrats. "I don't think they'd do that to the country."

The president appeared at the Capitol with House Speaker Thomas S. Foley (D-Wash.), other House leaders and four of 38 House Democrats who voted against the package the first time but have agreed to support the president on the final vote.

The four are Carolyn B. Maloney (N.Y.), Paul McHale (Pa.), Tim Johnson (S.D.) and Charles Wilson (Tex.). The Clinton budget passed by six votes in the House the first time through, and while Democratic leaders are short in a preliminary count of supporters, they are confident they will muster a majority.

Rep. Charles W. Stenholm (D-Tex.), a leader of conservative Democrats, announced yesterday that he would vote against the compromise package, in part because of constituent opposition to a gasoline tax increase of 4.3 cents per gallon.

Stenholm has played an influential role throughout the budget negotiations, and has done much to bridge differences between conservatives and liberals. But he complained that the compromise still doesn't provide enough in spending cuts, and said an executive order Clinton signed yesterday geared to restraining the growth in entitlement programs doesn't provide enough protection.

The White House pulled out all the stops to win over DeConcini after Sen David L. Boren (D-Okla.) withdrew his support. The Senate approved its version of the budget plan 50 to 49, after Vice President Gore cast a tie-breaking vote. Without a replacement for Boren, the bill would go down to defeat.

DeConcini, who cast critical votes in passing the Panama Canal treaty in 1978 and rejecting the Supreme Court nomination of Judge Robert H. Bork in 1987, was chary of backing a bill that raises the gasoline tax and taxes on corporations, wealthier Americans and some Social Security beneficiaires.

He relented, however, after the administration and Democratic budget negotiators agreed to water down the proposed Social Security tax increase and after Clinton agreed to issue an executive order, signed yesterday at a ceremony attended by DeConcini, creating a deficit-reduction fund.

One executive order -- derided by Republicans as a gimmick -- would segregate the projected $496 billion to be trimmed from the expected deficit into a separate account in order to protect the money from being raided by Congress for new spending or lower taxes. But because the president cannot order Congress to use the new tax revenue to reduce the deficit, the order remains largely symbolic -- at best, raising the political ante for Congress if it decides to dip into those funds.

The second executive order, along the lines of Stenholm's proposal, seeks to limit the growth of spending on entitlement programs such as Medicare and Medicaid, food stamps and Aid to Families with Dependent Children, and other mandatory spending programs whose fast growth is eating up an increasing share of federal spending.

Under the order, if the growth in entitlement spending exceeds estimated amounts, the president must report to Congress on the size of the problem and the steps, if any, he proposes to correct it. But, as with the trust fund, Congress retains the flexibility to act as it wishes.

Overnight polls by news organizations rendered a split decision on Clinton's speech. Polls by NBC and CNN-USA Today found that a plurality still opposed the economic program, while a CBS survey found a plurality now favors it. But the CBS poll also showed that only 29 percent of those surveyed approve of Clinton's handling of the economy.

To counter impressions that Clinton's speech had not moved voters, administration officials released the results of their own poll, commissioned by the Democratic National Committee, which showed a plurality favoring the plan, 48 to 41 percent.

More than half of those surveyed still believe the tax burden falls heavily on the middle class, but presidential pollster Stan Greenberg said the percentage -- which he declined to provide -- had dropped 9 points as a result of the president's speech.

Staff writer Dan Balz contributed to this report.