Bess Truman, by way of my dad -- and everyone else who knew about it in the 50's.
Tried to verify it by checking a history of Zionism that I found in a public library on the returns cart (I was there looking for something else) -- the author verified that the meeting took place, but glossed over what was said.
The author said the gentlemen -- three of them from the AJC and iirc the WJC -- came to reason with Truman, on the basis that if only the President could be acquainted with the facts, he would surely extend the recognition the new Israeli state so desperately needed. He didn't mention the part about the political contributions of the Jewish community going en masse to Tom Dewey instead of HST, if the President didn't come across.
That was the transaction. And Harry was hopping mad and used all sorts of unprintable language afterward, but they had him and they knew it. They got what they wanted.
That's the story. But I'm sure you've heard it.