West German Chancellor Helmut Schmidt urged his Social Democratic Party today to reject a move for a freeze in plans to deploy new nuclear missiles in Europe, arguing that a moratorium would take the pressure off the Soviets to negotiate reductions in their own arsenal in talks under way in Geneva.

A national convention of Social Democrats meeting here is due to vote Thursday on a proposal favored by the party's active left wing advocating a stop in plans to station Pershing II and cruise missiles in Europe while the Geneva negotiations go on.

Party leaders expect that the motion could draw support from as many as one-third of the 400 delegates and give encouragement to West German protest groups intent on undercutting Bonn's commitment to NATO policy.

This week's party conference comes amid mounting speculation that Schmidt's left-center coalition, only 18 months into a four-year term, would not last the year. A severe loss of public confidence in the Social Democrats was confirmed in two recent provincial elections.

In a toughly worded defense of his government's achievements that included a warning about the setbacks to Social Democratic aims that would result from a conservative government, Schmidt urged an end to divisiveness in his party and called for a "major new strengthening effort."

He said 1982 would be "a year of fateful decisions" for the Bonn government, with some critical moments both internationally and domesticly still ahead. Acting more the supplicant than his usual role as impatient, lecturing leader, Schmidt pleaded for party backing.

"I ask you to help me," he said. He then added quietly, "To tell the truth, one sometimes feels very alone with the responsibilities of state."

The chancellor seemed intent on avoiding antagonizing leftist factions in the party where he could. On economic policy, he appeared ready to concede to a widespread party wish for more state spending and higher taxes on the rich to finance a jobs-creation program for West Germany's rising number of unemployed.

He attacked neoconservative economic programs--as practiced by the Reagan administration and Britain's Margaret Thatcher--as creating more unemployment without achieving new growth or stability. But Schmidt sought to fight off leftist challenges to the convention's other two major issues: the NATO missile decision and a proposed two-year halt to construction of nuclear power plants in West Germany.

Spelling out the rationale for the NATO plan, Schmidt said the Soviets must be convinced that the West will deploy new missiles before Moscow can be expected to agree to reduce its own. He said the deployment program should not be dropped unless the Soviet Union "renounces its incredible armada of SS20" medium-range nuclear missiles, estimated by Western officials now to number 300.

To lessen chances of a confrontation on the NATO plan, the party's leadership has drafted a resolution that would postpone a yes-no decision on the missile deployment until a special party conference to be held nearer to the scheduled start of the stationing in December 1983.

Although Schmidt would prefer a firmer party endorsement of the NATO plan now, the postponement at least sidesteps the matter of his year-long threat to resign if the party reverses the support it gave the plan at a 1979 party convention.

Schmidt's position was bolstered yesterday when party chairman Willy Brandt, more popular with the left than Schmidt, said there should be no "backing away from the NATO decision that could give the Soviet Union an alibi for not negotiating seriously."

At the same time, equivocating somewhat on the official Western position, Brandt said there need not be an "automatic" link between eventual deployment and progress in the negotiations.