The latest Washington Post-ABC News Poll is full of bad news for President Reagan and the Republicans, which is hardly a surprise, given what has been happening to the economy. But there is one finding that sounds a cautionary note for Democrats. When people were asked about the alternatives to Reagan's budget, the inclination was to give the Democrats a big fat zero.

Only 26 percent said they had heard a better Democratic alternative, while 11 percent said the Democratic ideas were worse, and 43 percent said the Democrats were offering no alternatives at all.

These were the same people who were saying, by a 2-to-1 margin, that they want Congress to make substantial revisions in the Reagan budget. But, as far as most of them are concerned, there is a vacuum of ideas on the Democratic side.

Before the phone starts ringing with indignant calls from Democratic press secretaries and my mail is filled with fresh copies of the Democrats' latest speeches, let us stipulate that, this time, the public is wrong. There was a serious Democratic budget proposal on the table --and rejected--by the House last year. And there are outlines of alternative budgets coming from former vice president Walter Mondale and dozens of Democratic legislators this year.

But politics is a matter of appearance as well as reality. And the appearance that the Democrats are devoid of solutions for the nation's most critical problem is a significant political burden.

Every Democrat who has answered my questions about the implications of the poll has made the claim that without the White House, it is impossible to project a party policy position to the public. That claim is specious. It is a weak alibi.

The Republicans did not control the White House in the 1978-80 period when they established their patent on across- the-board tax cuts. They did not control the White House when they fixed the notion in the public mind that the Democrats were hooked on bureaucracy and soft on national defense.

They did that largely by using their House and Senate caucuses to draft and pass, usually by unanimous votes, manifestos on these subjects--and then putting the resources of the party into massive efforts to trumpet these positions to the voters.

In that period, the Republicans managed not just to exploit vulnerabilities of the Democrats, but to reconstruct the image of their own party in terms that made it easier for people to vote Republican.

Why have the Democrats so far failed to do anything like that? In part, it is because their congressional leadership, unaccustomed to the role of the opposition party, has been very poky about mobilizing for that kind of political effort. A year ago, the House Democratic leadership failed to rally support behind Budget Committee Chairman Jim Jones' alternative to the Reagan budget. The Senate situation was worse, with the top two Democratic leaders split on support and opposition to the Reagan budget.

Even in this year's radically changed conditions, the House and Senate Democrats have been speaking with many voices, and the public is understandably unaware of the serious and substantive proposals coming from congressional Democrats.

The other part of the failure must be ascribed to the Democratic National Committee, which has treated the issue area as its lowest priority. After much negotiation, a "Democratic Strategy Council" of federal, state and local officials met in Baltimore last October. As a get- aquainted session, the meeting went well. But the council did not meet again until February, when its one-day session was obliterated by the news of the release of the Reagan budget the same day.

Now it has been decided that the 100-member council is unwieldy, and four advisory panels are being formed from it to get more heavily into the production of policy. There's nothing wrong with that, but it's about a year late, and the starting time for new task forces is still uncertain.

There has been little communication or coordination between the Strategy Council and the separate policy task forces at work in the House and Senate Democratic caucuses. The Democratic National Committee has only three professionals working in the issues area --and a minuscule budget to publicize whatever positions the party might eventually develop.

It's no wonder the voters think that Democrats are devoid of ideas. What's more worrisome is that the ideas the Democrats are developing are not really being tested in serious intraparty debate. As one of those I talked to said, "We can win the next election on the Republicans' mistakes, but we can't govern on that basis."

I think the voters know that.