When Sen. Gary Hart (D-Colo.) went on the radio last Saturday to denounce as "a tragedy and a travesty" the Republican-sponsored budget-balancing plan that sailed through the Senate last week, it was billed as the Democratic response to President Reagan.

Actually, Hart spoke for himself and 19 other Senate Democrats who opposed the plan. He did not speak for the 27 Senate Democrats who joined the Republicans in passing it.

Three of the six Democratic senators who have sought the party's presidential nomination, including Edward M. Kennedy of Massachusetts, supported the GOP plan. Three others, including Hart, claimed it was "a political fraud," and voted against it.

The chairman of the Democratic Senatorial Campaign Committee, George Mitchell of Maine, opposed it, calling it "irresponsible and unacceptable." The chairman of the Democratic Congressional Campaign Committee, Tony Coelho of California, while favoring some changes, is arguing to fellow-Democrats that it would be smart politics to let it go into effect.

In short, what has happened is that the Republicans have rolled a political 10-strike with their deficit-cutting plan, and the Democrats are flying in every direction. With a single move, the GOP has taken the deficit issue, which seemed its greatest burden, and deftly shifted the monkey to the Democrats' back.

The substantive effects of the plan on both government programs and on the balance of power between the executive and legislative branches are important enough to merit separate comment. So many questions were left unanswered in the Senate's rush to judgment that it is well the Senate-House conference on the measure will be protracted.

But politically, it is evident already that the late-session initiative by two junior Republican senators, Phil Gramm of Texas and Warren Rudman of New Hampshire, has thrown the Democrats for a loop.

Beyond the obvious political posturing of such potential 1988 rivals as Kennedy and Hart, there was both intellectual confusion and fundamental strategic disagreement among Senate Democrats. Smart southern progressives such as Dale Bumpers of Arkansas and Lawton Chiles of Florida could not agree. Neither could canny conservatives such as Sam Nunn of Georgia and Bennett Johnston of Louisiana. Nor could liberal freshmen such as Paul Simon of Illinois and Tom Harkin of Iowa.

The disarray among House Democrats is, I suspect, going to be even greater, and show even more clearly the basic cleavages inside the old Democratic coalition.

The members of the black caucus and others representing urban and rural districts where government aid is a necessity, not a luxury, have been increasingly restive as the leaders of their party have accepted the philosophy of budget-cutting and avoiding a tax-increase.

They went along reluctantly with those, like Speaker Thomas P. (Tip) O'Neill (D-Mass.), who said Reagan would murder the Democrats politically if they came out for a tax hike, even as they saw aid to their cities and their constituents whittled away.

But this proposal -- which requires far deeper cuts and gives the president the power to impose them if Congress does not -- has ripped it. Rep. Ron Dellums (D-Calif.), one of those critics, charged that despite the support it drew from people such as Kennedy and Bumpers and Simon, "I see no guarantees (in the proposal) for the poor and powerless. The people that I represent who have been the most loyal supporters of the Democratic Party do not deserve to have their loyalty rewarded by abandonment."

Many white liberals, however, argue that the political imperative to "do something" about the deficit is irresistible, and they think Gramm-Rudman can be reshaped in conference to force Reagan to accept a choice between deep defense cuts and acquiescence in higher taxes.

They voted to go to conference on the measure despite deep misgivings about the proposal as it stands. Rep. John Dingell (D-Mich.) said his preliminary analysis suggests that the transfer of the power of the purse from Congress to the president is so great under the Senate-approved bill that "everything that's been gained (by legislatures in democratic countries) since the field of Runnymede has been thrown away."

Rep. David Obey (D-Wis.), another liberal conferee, said his hope is that "we can keep all categories of spending on the table, and then the military will have to take a real hit if Reagan refuses to consider a tax increase." But he, too, conceded that "at this point, nobody knows what's in it and what it would really do."

Backstage there are Democratic strategists who don't really care what is in it, or what it does, so long as it is rewritten to take effect in 1986 and so long as the voters know it's basically a Republican design.

House Democratic campaign chief Coelho argued that "when the crunch comes, and programs people want are cut, they will remember who pushed it."

That hope of a public backlash is little consolation to those like Dellums who see Gramm-Rudman as the final step in Reagan's design to dismantle the welfare state. They see betrayal, not shrewdness, in the acquiescence of so many Democrats in that plot.