President Harry Truman supported the Zionist movement because he believed the international community was obligated to fulfill the promise of the Balfour Declaration and because he believed it was the humanitarian thing to do to ameliorate the plight of the Jewish survivors of the Holocaust. He did not believe the rights of the Arabs should or would be compromised. A sense of his attitude can be gleaned from a remark he made with regard to negotiations as to the boundaries of a Jewish state:
The whole region waits to be developed, and if it were handled the way we developed the Tennessee River basin, it could support from 20 to 30 million people more. To open the door to this kind of future would indeed be the constructive and humanitarian thing to do, and it would also redeem the pledges that were given at the time of World War I.
The American public supported the President's policy. According to public opinion polls, 65% of Americans supported the creation of a Jewish state. During the third quarter of 1947 alone, 62,850 postcards, 1,100 letters and 1,400 telegrams flooded the White House, most urging the President to use American influence at the UN.
This public support was reflected in Congress where a resolution approving the Balfour Declaration was adopted in 1922. In 1944, both national parties called for the restoration of the Jewish Commonwealth and, in 1945, a similar resolution was adopted by Congress.
Rather than giving in to pressure, Truman tended to react negatively to the "Jewish lobby." He complained repeatedly about being pressured and talked about putting propaganda from the Jews in a pile and striking a match to it.
When the UN took up the question of Palestine, President Harry Truman explicitly said the United States should not "use threats or improper pressure of any kind on other delegations." Some pressure was nevertheless exerted and the U.S. played a key role in securing support for the partition resolution. U.S. influence was limited, however, as became clear when American dependents like Cuba and Greece voted against partition, and El Salvador and Honduras abstained.
Many members of the Truman Administration opposed partition, including Defense Secretary James Forrestal, who believed Zionist aims posed a threat to American oil supplies and its strategic position in the region. The Joint Chiefs of Staff worried that the Arabs might align themselves with the Soviets if they were alienated by the West. These internal opponents did a great deal to undermine U.S. support for the establishment of a Jewish state.
Although much has been written about the tactics of the supporters of partition, the behavior of the Arab states has been largely ignored. They were, in fact, actively engaged in armtwisting of their own at the UN trying to scuttle partition.
Sources: Zionism and Israel Center