French Premier Survives Vote : Leaders: Last-minute support from moderate rightists helps Rocard turn back a challenge. Thatcher faces her test today.

French Prime Minister Michel Rocard, whose minority government faced loss of a censure vote in the National Assembly that would have forced him to resign, won last-minute support from moderate right-wing parties late Monday night to survive the most serious challenge of his two years in office.

Rocard was the first of two key European heads of government threatened with the embarrassing prospect of losing their jobs while a 34-nation summit of world leaders, the Conference on Security and Cooperation in Europe, meets here in Paris.

Today, British Prime Minister Margaret Thatcher will be tested when Conservative Party members of Parliament vote on a challenge to her party leadership from former Defense Secretary Michael Heseltine. If Thatcher loses, she would have to step down from the prime minister’s post she has held for more than 11 years.

The last British leader to lose his post during an international meeting was Prime Minister Winston Churchill during the Potsdam conference in 1945. Despite the pressure, Thatcher was the picture of poise and confidence Monday morning as she met with reporters at the British Embassy in Paris.

“I most earnestly believe I shall be at No. 10 Downing at the end of this week,” she said, referring to the prime minister’s official residence in London.

Meanwhile, the short, combative French leader Rocard was being credited by supporters with “Churchillian” courage as his minority Socialist Party leadership overcame its ninth censure vote since it was installed after parliamentary elections in 1988.

This time the challenge was based on opposition to a 1.1% income tax Rocard has proposed as part of a reform of funding for Social Security programs.

Since the Socialists do not have an absolute majority in the 577-member National Assembly, Rocard needed 17 votes or abstentions from other parties to survive the censure vote. In the past, the Socialists had always been able to count on the 26 Communist Party deputies in the Assembly to back them in censure votes, the French equivalent of a parliamentary vote of confidence.

However, this time the Communists decided to vote for censure, forcing the Socialists to find the needed votes among independents and members of center and center-right parties. In the end, the Socialists survived the vote by persuading six right-wing deputies, mostly from the French overseas territories, to break ranks with their parties. The opposition mustered 284 votes, five short of the absolute majority needed.

Rocard, 60, never gave signs of losing confidence. “I approach this vote with serenity,” he said. However, he gave a deep sigh of relief when National Assembly Speaker Laurent Fabius read the vote results before the packed Parliament.

If Rocard had lost the vote, French President Francois Mitterrand would have had to appoint a new prime minister or call for new elections. Mitterrand has been criticized for not giving longtime Socialist Party rival Rocard enough support during the censure vote debate.

But the French president, 73, whose seven-year term will not expire until 1995, is also known to dread the possibility of elections leading to a right-wing opposition majority in Parliament, forcing him to install another so-called cohabitation government like the one he shared with Gaullist Prime Minister Jacques Chirac in 1986-1988.

Because he has been forced to govern without a majority, Rocard has been under constant challenge since his appointment by Mitterrand two years ago.

Thatcher, however, is the longest-serving national leader among the 34 CSCE member countries meeting in Paris. She became party leader when she deposed Edward Heath in 1975, when the Conservatives were in opposition, and became prime minister in 1979.

Unlike the opposition Labor Party, the Tory party rules provide for annual elections to determine the party leader. And whoever is voted leader traditionally becomes prime minister if the party is in power.

British public opinion polls published Sunday and Monday show Thatcher trailing Heseltine--among voters--by a considerable margin.

However, Tuesday’s balloting is restricted to the 372 Tory members of the House of Commons.

In interviews Monday, the prime minister accused Heseltine of holding political views indistinguishable from those of the Labor Party.

For his part, Heseltine contends that Thatcher is only lukewarm in supporting a strong role for Britain within the European Community.

Asked at a press conference at the British Embassy here Monday if she would be returning home to a new address, the prime minister said she expects to stay in office.

Why is she so confident?

“I have a marvelous team working with me,” she replied. “We are all confident.”

Is she under a strain?

“Not in any way. I’ve been attending conferences like these for 11 1/2 years. It’s an almost everyday occurrence.”