This weekend, the Democrats get their say. Up against a multi-million-dollar Republican Party advertising campaign and the publicity machine of Ronald Reagan's White House, the Democrats are trying to strike a blow for fairness and balance in the public debate.

At least that's what they'll tell you, as they try to prepare the press and public for the blast of Democratic Party propaganda that will be coming down from the mid-term mini-convention in Philadelphia Friday through Sunday.

Half-a-dozen prospective presidential candidates, some of the House and Senate party leaders and many prominent governors and mayors will give testimony in Philadelphia to the ineffectiveness and unfairness of the Reagan economic and social program. At a higher level, perhaps, they will identify the party with the growing movement to demand a freeze on further nuclear weapons development and deployment.

Some of what they say will be on target. But what they won't talk about is also relevant. And by far the most important topic not on the agenda is the way the Democrats' disunity has contributed to policies and problems they decry.

Obviously, it took Democratic votes to pass Reagan's budget--with its weapons buildup and social-program cuts--and Democratic votes to pass Reagan's tax bill in the House of Representatives, because Democrats control the House. What is forgotten is that, less than a year ago, majorities of the Democrats in the Senate voted for passage of all three of the landmark Reagan economic bills: the first budget, the reconciliation bill carrying out its provisions, and the tax bill.

It is true that Democrats offered many amendments aimed at changing Reagan's plans. But when those amendments failed, as almost all of them did, large majorities of the Senate Democrats bowed to public and political pressures and gave Reagan what he wanted.

The votes among Senate Democrats were 28-18 in favor of the budget; 28-15 in favor of the reconciliation bill; and 37-10 in favor of the tax bill.

All five of the Senate's presidential hopefuls, Alan Cranston, John Glenn, Gary Hart, Ernest F. Hollings and Edward M. Kennedy, can honestly say they opposed the first Reagan budget. On the reconciliation bill, however, Glenn and Hollings slipped away. And on the tax bill, Cranston and Glenn voted with Reagan.

More important than their individual votes is the question of collective party responsibility for the policies that are now being condemned. It is not an academic question, although Democrats are doing their best to keep it off the Philadelphia agenda.

The Democratic leader of the Senate, Robert Byrd of West Virginia, who is not scheduled to attend the Philadelphia meeting, voted with Reagan on all three of the major economic measures of 1981. No Democrat, to my knowledge, has suggested publicly that this reflects on Byrd's leadership. But one can imagine the outcry among Republicans if Byrd's counterpart, Senate Majority Leader Howard Baker had opposed Reagan on those three measures.

In the House, about one-quarter of the Democrats have voted with some regularity for Reaganomics. None has been disciplined or even chastised by the leadership.

Earlier this month, there was a most instructive test case. The most prominent northern Democrat-for-Reagan, Rep. Ronald M. Mottl (D-Ohio), was challenged in a Cuyahoga County (Cleveland) primary by a prominent local elected official, who argued that Mottl had flunked a minimal test of party loyalty. Although the challenger was supported by the local Democratic organization, the House Democratic leadership arranged for Mottl to get a $5,000 contribution from the Congressional Campaign Committee.

House Speaker Thomas P. O'Neill, the Philadelphia convention keynoter, praised Mottl publicly before the primary, and when Mottl lost on June 8, turned around and said it was a "good lesson" to other Democrats.

But the lesson has obviously not been learned. Two days after that primary, 39 House Democrats voted against the Democratic budget proposal, defeating the measure that would have rolled back the 198l tax losses and saved some domestic programs. Then, 46 House Democrats voted for the Reagan-Republican budget, enabling it to pass.

But in the Senate, the Democrats in 1982 found "their courage." With Byrd in the forefront, they held out, 41-3, against the Republican budget.

Listen closely to see if anyone in Philadelphia tries to rationalize that record. But don't hold your breath waiting.