While promising to hold national elections in March, Helmut Kohl's new West German government continues to face legal and political uncertainties over arranging the vote and surviving it intact.

A key stumbling block remains Kohl's junior coalition partner, the troubled Free Democratic Party. Outspoken resistance to early new elections emerged last week from former president Walter Scheel, a senior Free Democrat.

Another, Guenther Verheugen, the party's former general manager, announced unexpectedly Thursday that he was quitting to join the opposition Social Democrats. He is the fourth deputy to leave the small party's originally 53-member caucus since the Free Democrats abandoned the Social Democrats in September to form a new coalition with Kohl's Christian Democratic Union.

Verheugen had resigned as party manager earlier this month in protest over the government change and, at a national congress in West Berlin two weeks ago, he led an unsuccessful left-wing challenge of party Chairman Hans-Dietrich Genscher's shift to the right on economic and security policy. Still, Verheugen had been widely expected to stay in the party.

"The Free Democratic Party has become a mere majority maker for the Christian Democrats," Verheugen, an ex-protege of Genscher, told reporters. "It was clear to me after the Berlin congress that I had no political home in the Free Democrats."

Verheugen's switch precedes a rump convention Sunday in Bochum of other disenchanted left-wing Free Democrats who are considering the establishment of a new, breakaway party. Some leading left-wing Free Democrats, notably former interior minister Gerhart Baum, have urged dissidents from the left wing to remain in the party to act as a pressure group on the right wing. But party officials report a loss already of 6,000 members, down to a total of 80,000, in recent weeks.

Meanwhile, with just three months to go before the tentative March 6 election date, Kohl has yet to make clear how he intends to bring the vote about.

The West German constitution does not allow a chancellor to summon new elections or the Bundestag (lower legislative house) to dissolve itself.

Nonetheless, the Christian Democrats and Free Democrats, when they formed a new right of center coalition eight weeks ago, agreed to schedule early elections in view of public disapproval of the parliamentary tactics used to remove the old government led by Helmut Schmidt. The next regular elections were not due until 1984.

This has created a problem for Kohl. Attempting to spread responsibility, he has discussed options privately with other party leaders and with President Karl Carstens. The president, normally just a figurehead, is the only one empowered -- under specific constitutional conditions -- to dissolve the Bundestag.

Kohl could propose amending the constitution to give the Bundestag the right to dissolve itself, but he is understood instead to favor bringing new elections by deliberately losing a confidence vote in parliament. Prospective dates for the vote range from mid-December to early January.

Carstens, a Christian Democrat, has declined to state publicly whether such a procedure will receive his approval.

But former president Scheel raised strong objections to the plan, arguing that it would involve a manipulation of the constitution, because the Bundestag was not meant to be dissolved as long as a workable political majority existed.

Some Free Democratic deputies reportedly intend to challenge in federal court the constitutionality of the confidence-vote maneuver if Kohl tries it. This could result in a court injunction against the March election date or their cancellation.

If elections were held soon, national polls suggest that the Free Democrats would fail to get the 5 percent of the vote required for representation in parliament.

The opposition Social Democrats -- whose proven vote-getter, Schmidt, recently declined to run again -- also would seem to have an interest in putting off a national vote. But they are firmly committed publicly to the March 6 date.

Two independent opinion surveys published this week, one by the Allensbach Institute and the other by Infratest, indicate that the Christian Democrats currently would draw between 49 and 51.1 percent, the Social Democrats 38.6 to 42 percent, the Free Democrats 3 to 3.6 percent and the radical left Greens 5 to 6.3 percent.

The good news for Kohl in these figures is not only that his party remains within reach of an absolute majority, but also that the Greens appear to be losing supporters. They were drawing nearly 10 percent in polls last summer.