Marlin Fitzwater, the President's spokesman, said the site of today's ceremony was selected for political reasons, since the Chicago area has a large Jewish population with a ''special interest'' in the genocide bill. The ceremony, which had to be moved into a hangar because of heavy rain, was attended by two dozen prominent Jewish and political figures.

The Genocide Convention was approved by the United Nations General Assembly on Dec. 9, 1948, and submitted to the Senate by President Truman in June 1949. It languished in the Senate for many years, even though Senator William Proxmire, a Wisconsin Democrat, took to the Senate floor every day that it was in session for 19 years and gave more than 3,000 speeches to urge its adoption. Mr. Proxmire, who is retiring this year, was invited to the signing ceremony but could not attend, the White House said. Senate Consent in 1986

The Senate finally consented to ratification on Feb. 19, 1986, with the reservation that legislation be passed that conforms American law to the terms of the treaty.

The enabling legislation was approved by Congress last month, and the signing today put it into effect.

The law defines genocide as acting with a ''specific intent to destroy, in whole or in substantial part, a national, ethnic, racial or religious group.''