Less than four weeks from March 6 elections, West Germany's Social Democratic Party is staging a startling political comeback by capitalizing on public opposition to new nuclear missiles and rifts within the ruling coalition to close the gap with the Christian Democratic Union.

Displaying a new spirit of unity through the calm, conciliatory style of their candidate for chancellor, Hans-Jochen Vogel, the Social Democrats have rebounded in the polls from below 30 percent six months ago to about 44 percent today, only three points behind the governing Christian Democrats.

Aided on the campaign trail by former chancellors Willy Brandt and Helmut Schmidt, Vogel has shrewdly focused his appeal on themes designed to recapture votes on the left and gain new ground toward the center.

He has courted antinuclear elements who flocked to the Greens movement by stressing efforts to deter installation of new cruise and Pershing II missiles and to protect the environment.

At the same time, Vogel has emphasized themes like the need for more jobs and social justice to woo dissident members of the Free Democrats who were dismayed by the party's decision to abandon a 13-year ruling partnership with the Social Democrats to enter into a new coalition with the conservatives.

As the campaign reaches a climax, Social Democratic strategists believe that they have gained enough momentum to convince the voters that the election will be decided between the major parties and that the Greens and the Free Democrats do not have enough support to sustain the 5 percent requirement to hold seats in the Bundestag, the lower house of parliament.

Recent polls indicate that the Greens have fallen to 5 percent or less from a high of 9 percent last September, while the Free Democrats, or liberals, have not increased their votes much above 4 percent. If these trends continue, the Social Democrats believe, then voters will prefer to cast their ballot where it counts with one of the bigger parties.

Brandt, an early advocate of closer ties between the Social Democrats and the Greens, now says he feels "as good as certain" that the antinuclear, ecological group will not enter parliament.

Sensing their vulnerability, the Greens have moderated their demands and admit they could support a Social Democratic government if it vows to halt nuclear energy projects and deployment of the missiles.

But even though Vogel could probably satisfy those conditions without much difficulty, the Social Democrats have balked at cutting a deal with the Greens and now intend to strive for a parliament consisting only of the major parties.

"We're quite sure that the liberals and the Greens are not going to make it," said Peter Glotz, the Social Democrats' campaign manager, in an interview. "Our chances are only 3 to 1 to become the strongest party, but we have a good shot if the Christian Democrats continue to make mistakes."

Chancellor Helmut Kohl's campaign has been hampered by signs of confusion and dissension among the coalition parties over economic strategy and support for President Reagan's zero option proposal to cancel deployment of nuclear missiles in Europe if the Soviet Union agrees to scrap its countervailing arsenal of medium-range missiles targeted on Western Europe.

Kohl endorsed the Reagan line but his foreign minister, Free Democratic leader Hans-Dietrich Genscher, soon called for an "interim solution" of partial missile forces on both sides until ultimate elimination could be negotiated.

Later, Franz Josef Strauss, the head of the Christian Social Union, the Bavarian sister party of the Christian Democrats, described the zero option as "absurd" but said he favored deployment of the new missiles. Then Kohl also admitted the zero option was not sacrosanct by claiming it was "not an all-or-nothing approach."

The coalition partners also have squabbled over taxes and investment policy. The Free Democrats want to reduce the taxes on small business firms but the Christian Democrats, sensitive to charges of "protecting the rich," have sought to spread the burdens of austerity plans.

The Christian Democrats, however, have maintained their lead in the polls largely by pounding home the message that West Germany's economic ills derive from spendthrift budgets during the Social Democrats' long reign in power.

In addition, they have warned that if Social Democrats teamed up with the Greens to form a "Red-Green Alliance" in the next government, West Germany's economy would suffer massive capital flight and a severe loss of confidence in the deutsche mark.

Recently, Gerhard Stoltenberg, the Christian Democratic finance minister, declared that many business contracts lately were drawn up with an "election clause" to allow companies to cancel investment plans if the conservatives were not returned to power.

The Social Democrats launched a leaflet drive accusing the Christian Democrats of encouraging an "investment strike" if the Social Democrats were victorious in the March 6 election.

Kohl responded to the attack by urging all companies to accelerate their investments regardless of election considerations, though his spokesmen chastized the Social Democrats by saying they should ask themselves why business firms were holding back.

After holding open the prospect of ruling with the tacit support of the Greens, Vogel and the Social Democrats now say they want to keep the Greens and the Free Democrats out of the Bundestag. "We never wanted a real coalition with the Greens," says Glotz. "We always knew it would be impossible to rule with them."

Recognizing that their hopes for winning the election are slim despite their resurgence, Social Democratic strategists say they are not frightened by a possible absolute majority for the Christian Democrats.

They believe Kohl would ultimately be dragged down by his rivalry with Strauss and the lingering economic malaise, while the Social Democrats would gain time to rebuild a new majority from sympathetic remnants of the Greens and Free Democrats.

The worst case for the Social Democrats, says Glotz, would be a Christian Democratic victory with the Free Democrats staying in parliament. "If that happens," he says, "then we may be stuck in the opposition for more than a decade during a long era of center-right government."

Vogel, Brandt and other Social Democrats are expected to intensify their attacks on the Free Democrats to thwart any chance that they might surpass the 5 percent mark and retain their parliamentary seats.

The Social Democrats, while admitting their chances to emerge as the strongest party remain uncertain, are banking on their traditional success in swinging the margin of victory in the campaign's final weeks.

To do so, they feel they must capture new voters from working class Christian Democrats who are disgruntled by high rents and reduced subsidies, cut the Greens' share of the vote down to 3 percent and proselytize disaffected liberals into casting their ballots against the notion of a coalition with conservatives.

The Social Democrats also hope to turn Kohl's economic arguments against him by claiming that 700,000 people have joined the ranks of the unemployed since he became chancellor four months ago.

In that sense, the election outcome may hinge on whether the voters decide to pin blame for West Germany's record unemployment levels, which may exceed 3 million later this year, on the Christian Democrats or their rival predecessors.