The United States has thrown its support behind Jacques de Larosiere, the leading European candidate for managing director of the International Monetary Fund - bolstering his position as frontrunner for the job.

W. Michael Blumenthal, the U. S. Secretary of the Treasury, has publicly endorsed de Larosiere, director of the French treasury and a close friend of French President Valery Giscard d'Estaing - the first time the Carter administration has entered the foray.

Blumenthal told reporters earlier this week the U. S. "would be very happy" to see de Larosiere chosen as the next IMF managing-director. He said the 48-year-old Frenchman would be "a very acceptable candidate from our point of view."

The Secretary's remarks appeared likely to bolster de Larosiere's Witteveen in the top IMF job. The French already are pushing hard for his appointment, and the European Common Market has backed him - though not unanimously - as its "Main" candidate.

Witteveen announced late in September he would step down from the managing-director's post next summer, mainly for personal reasons. The former Dutch finance minister has been head of the Fund since 1973, following the resignation of Pierre-Paul Schweitzer.

The IMF post traditionally has gone to a European, as the presidency of the Fund's sister organization, the World Bank, always has been reserved for an American. The present head of the World Bank is Robert S. McNamara, former Johson administration Secretary of Defense.

Several developing nations have been campaigning privately this time to put a third-world candidate at the head of the Fund, but they are not expected to succed. Another European time finance minister of the Netherlands.

De LSarosiere, a career civil servant who was brought to the number-two French treasury post by Giscard d' Estaing in 1974, is described as a skilled behind-the -scenes bargainer who essentially is a conservative in monetary affairs.

In his first year in office, he played a major role in negotiating the compromise between France and the U. S. that led to completion of the four-year quest for overhaul of the previous world monetary system. He was regarded by U. S. officials then as a nemesis.

The de Larosiere candidacy is expected to come up formally at a meeting of the Common Market's finance ministers December 19, when the group is scheduled to decide whether to endorse him or someone else. The EEC is seeking a joint approach to head off any contest.

Technically, the decision on the new managing-director is in the hands of the IMF's board of directors. However, the recommendations of European governments and the United States have traditionally dictated the choice. It was U. S. insistence in 1973 that forced Schweitzer to step down.