Robert J. Dole, the Senate majority leader, is bravely marching forward to try to pass a budget that actually reduces the deficit. "I think it's very important," he said last weekend. "No one else seems to think so, but I do." There the senator was needling some of his friends -- including his friends at the White House. Everybody thinks that the budget is important, but a lot of people evidently think that other things are more important.

In reply to them, Sen. Dole argues that lower budget deficits will mean lower interest rates. The Reagan administration, justifiably sensitive on this point, has tried in the past to deny that any such relationship exists. But Sen. Dole is right. The most useful thing that Congress can do for the economy this year is prompt and orderly enactment of a budget that moves decisively toward lower deficits. That won't be easy.

When the voting begins in the Senate, Mr. Dole acknowledges, there are going to be some defections from the Republican majority. He candidly said on "Meet the Press" last Sunday that he will need help from some of the Democratic senators to get the budget through. But why would Democrats support the Republican budget as it now stands?

Under the Republican budget, defense spending would rise less rapidly than Mr. Reagan originally wanted -- but it would rise. To help finance it, there would be a pretty substantial nick in Social Security benefits. A better way to pay for it would be a tax increase, but the White House was adamant. Why would Democrats vote for a budget that goes after Social Security but refuses to raise revenues?

Mr. Reagan is to go on television this evening to support that budget. If he launches into a vehement attack on all those senators who would cut defense, preserve pensions or raise taxes, the prospect for the next round of compromise with the Democrats will be poor. Lawton Chiles, the ranking Democrat on the Senate Budget Committee, has put forward a counterproposal that goes a long way to accommodate the Republicans. But it would reduce the erosion of Social Security checks, and it would require some tax increases. The Democrats would be quite right to insist on that, to accompany the spending cuts for which they are being asked to vote.

This week will demonstrate Mr. Reagan's order of priorities -- whether he thinks a budget working toward lower deficits is crucial, or whether other causes and debating points are more important. If he wants to get his deficits under control, this year will probably be the last chance of his presidency. And, as Sen. Dole says, he is going to need Democratic votes to do it.