Vice President Bush joined other leading conservative politicians from 19 countries today in launching the International Democrat Union, an organization, Bush said, to "carry on the great work of promoting democracy."

British Prime Minister Margaret Thatcher, West German Chancellor Helmut Kohl and the prime ministers of Denmark and Norway were on hand. They were among representatives of parties in 14 European countries, Japan, Australia, New Zealand, Canada and the United States that are members of the group.

In addition to Bush, the United States was represented by Republican National Committee Chairman Frank J. Fahrenkopf Jr. His Democratic counterpart, Charles T. Manatt, was present, but only as "an observer."

For the Europeans especially, such intraparty organizations are nothing particularly new. The Socialist International is a longstanding group with members from 40 countries who meet regularly, and European conservatives formed a similar organization in 1978.

But American participation reflects an initiative that began with the Reagan administration and has drawn some support from Democrats as well for a more systematic effort to bolster democratic parties around the world. In his remarks today, Bush recalled that President Reagan pledged such an effort in a speech to the British Parliament a year ago.

The International Democrat Union's chairman, Alois Mock of the Austrian People's Party, picked up that theme in his opening statement, asserting: "Political ideas have no national frontiers, and we will not be slow to export ours. For too long many of us have left the promotion of ideas to the left."

Bush, Thatcher, Kohl and other speakers stressed the contrast between their own democratic societies and the repression in communist nations. "Ten years ago," Thatcher said, "people were writing about the crisis of democracy. Today attention has turned to the crisis of state socialism."

The vice president singled out U.S. support for the "free nations" of Central America, which he said are receiving "the aid they need to build and perfect democracies of their own."

Speaking at the start of an eight-nation European trip in which he will explain administration policy in Central America, Bush pointed out that U.S. economic assistance to the region is three times as great as the military aid earmarked to help the countries "resist attempts at subversion by forces beholden to totalitarian powers."

Later, at a press conference, Bush acknowledged that many Europeans, including some from countries represented in the new conservative organization, are uneasy about U.S. intentions in Central America. "Our objectives are honorable," Bush said. "We just haven't gotten out as well as we should what motivates this president in Central America."

This morning Bush conferred with Thatcher and her new foreign secretary, Sir Geoffrey Howe. Saturday he flies to West Germany and later will visit other countries including Scandinavia and Ireland.

The union brings together the European Democrat Union and a similar group formed last year by the Pacific countries. It will be based at the headquarters of Britain's Conservative Party and in time is likely to be one of the agents of the U.S. program to boost democratic parties. A bill for the establishment of a National Endowment for Democracy has been passed by the House, although its exact nature is still not fixed.