Democratic Party leaders put aside the differences of the Vietnam war era and agreed today on the need to strengthen conventional forces against the Soviet threat, while curbing the arms race through a nuclear weapons freeze or other negotiated agreement.

The participants in the midterm national party conference workshop on national security also brushed aside any criticism of the Israeli invasion of Lebanon and instead said the bloody fighting has created "a new opportunity to build a lasting peace for the people of Lebanon and greater security for Israel."

The same show of harmony prevailed in the economic policy panel, where condemnation of President Reagan's policies was combined with endorsement of a hybrid "tax-streamlining" proposal that would end most deductions and move toward a flat-rate tax but preserve some progressivity.

"There is nothing like being out of office to encourage pragmatism," commented Sen. Paul Tsongas of Massachusetts, who ran the foreign policy panel.

The only raucous note on the middle day of the weekend mini-convention, which brought about 900 delegates and 5,000 others to the City of Brotherly Love, came from three groups of demonstrators outside the Civic Center.

Picketers from the National Organization for Women (NOW) threatened political retribution against Democratic legislators who helped block the ratification of the Equal Rights Amendment. Members of ACORN, a community-based populist organization, carried their protest against the alleged exclusion of poor people from the Democratic meetings into a hotel fund-raiser for Democratic governors. And followers of Lyndon LaRouche condemned the policies of elder statesman Averell Harriman.

Inside the meetings of the seven issues panels, harmony was the watchword. Tsongas' session voted unanimously for a slightly strengthened resolution saying "the Democratic Party welcomes and supports the national movement to control nuclear weapons and prevent their use, including the national nuclear weapons freeze campaign, as a clear expression of the strong desire of the American people to halt and reverse the nuclear arms race."

Even after "freeze" supporters from Vermont and Wisconsin argued successfully for the addition of the words "and supports" to the draft text, it was accepted by traditional "hawks." In return, Vietnam-era "doves" readily agreed to an AFL-CIO amendment specifying that any nuclear arms agreement must be "consistent with the maintenance of overall parity" with the Soviets.

Under the rules, Sunday's closing plenary session will vote to accept the workshop policy statements but cannot amend them.

Tsongas said the unusual harmony in a party that has been fractured by foreign policy differences for the past 15 years showed "a picture of Democratic unity . . . in sharp contrast to the disarray" in the Reagan administration symbolized by Friday's sudden resignation of Secretary of State Alexander M. Haig Jr.

Although Tsongas had voted against Haig's confirmation, the Foreign Relations Committee member said the resignation had removed "the most effective voice for moderation and stability in the inner councils . . . The hardliners have triumphed."

Neither hardliners nor Tsongas' kind of anti-Vietnam liberals could claim victory in the foreign-defense policy manifesto, where each call for nuclear disarmament was balanced by an acknowledgment of a need to counter with conventional forces a "Soviet military buildup . . . continuing beyond any reasonable justification."

Asserting that "the Soviet buildup requires a measured response, free of exaggeration or underestimation," the statement contrasted a Democratic policy of "steady, substainable modernization of our military forces" with a Reagan policy it characterized as "spasms of spending without plan or purpose."

Echoing a favorite theme of Sen. Gary Hart (D-Colo.), the only presidential hopeful to join in today's discussion, the statement said a buildup of conventional forces is as "imperative" as strengthened efforts to control and reduce nuclear arms.

In the arms control area, it endorsed a variety of steps, including, in an amendment from the floor, immediate ratification of the SALT II agreement. But most attention focused on the "nuclear freeze" motion for potential intenal conflict.

But the draft statement, which Democratic National Committee aide Bernard Aronson had negotiated with potential candidates' representatives and spokesmen for such opposing groups as the AFL-CIO and Americans for Democratic Action, held up under the stress of the morning's debate in the sweltering convention center.

Tsongas said the language is broad enough to embrace both the nuclear freeze resolution popularized by Sen. Edward M. Kennedy (D-Mass.) and the nuclear reduction resolution cosponsored by Sen. Henry M. Jackson (D-Wash.).

Allies of both -- Rep. Edward J. Markey of Massachusetts and writer Ben Wattenberg -- endorsed the proposal in debate today, even while disagreeing on the way it should be interpreted.

The careful crafting of the arms plank to close the historically deep divisions was in sharp contrast to debate on the Middle East resolution, where domestic political considerations were obvious and overriding.

It was drafted by Washington consultant Mark Siegel, a onetime Carter White House liaison to the Jewish community, and the major statements supporting it came from Reps. Toby Moffett of Connecticut and Michael D. Barnes of Maryland. Moffett is running for the Senate against Sen. Lowell P. Weicker Jr. (R), who claims strong Jewish support, and Barnes is opposed for re-election by Montgomery County School Board member Marian Greenblatt (R).

Moffett said "as a Lebanese-American," he supported Siegel's view that the Israeli invasion will contribute to "the reunification and restoration of Lebanese sovereignty and independence" as well as dealing "a severe blow" to "international terrorism" by the Palestine Liberation Organization and reducing "Soviet influence."

Even though the original resolution had commended "humanitarian relief efforts . . . to the civilian population of Lebanon," Christy Blackburn Rodriguez of Alexandria said she would have to oppose it because of what it did not say. "There are 600,000 refugees in West Beirut," she said, "facing a threat of typhoid. We cannot be silent on civilian deaths, no matter which side is responsible."

Rep. Mary Rose Oakar of Ohio also complained that there was "absolutely no mention of the thousands who were killed, no mention of the recall of [Israeli] troops."

Tsongas then suggested that some remarks Siegel had made in introducing the resolution be incorporated in its language. So the resolution was amended to read that "the Democratic Party states its regret at all loss of life on both sides." It then passed, with only a few murmured dissents.