I’ve now read (somewhat quickly) McCullough’s description of Truman’s decision process for recognizing Israel. I don’t see the quotation cited by Gergen, but it might be there. McCullough treats the subject in great detail, over fifty or so pages.
Truman may well have claimed, and might even have believed, that he did not decide to recognize Israel for political purposes, but we’ll never know. If politics were not important, why did he think it was so important that the US be the first to recognize Israel?
Furthermore, McCullough says one of the most important considerations was whether Secretary of State George Marshall would resign over the issue. If Marshall had resigned, Truman thought he would be doomed politically because Marshall was so highly respected. It was only after Marshall said that he would not resign over the issue that Truman felt that he could go further, pushed hard by the Jewish lobby.
Marshall told Truman that if he recognized Israel, it would be a reason for Marshall not to vote for Truman, because he felt that Truman was doing it for domestic political purposes. This was a strong rebuke to Truman, but toothless, because Marshall never voted. He felt that it would inhibit his ability to carry out his duties to his country if he chose political sides.
Unfortunately there is no one in government today with the character and stature of General George Marshall. Marshall was in large part responsible for the Allies’ victory in World War II. Then he was largely responsible for the US positioning itself to eventually win the cold war in the post-war world, sponsoring the Marshall Plan for the recovery of Europe, and perhaps avoiding a shooting war with the Soviet Union.