Harry Truman (D) saved an estimated 1 million G.I.s with that.
Truman didn't do it, FDR did. He had the entire project mapped out and orders signed before he died, including the order to bomb and keep bombing as more bombs became available.
Truman hadn't even been read on to the bomb project, so imagine his surprise when he's sworn in and right after he's told, "Oh by the way, we have a secret weapon that will end the war." All Truman did was stay out of the way of FDR's plan. The first three devices were the only ones that were hand-built. The rest were made on assembly lines. Some time around the end of 1945 (sources disagree on exactly when), the Washington reactors were making enough Pu-239 to make four more bombs every month. So come January of '46, if Japan hadn't surrendered, they could have been receiving another atomic bomb every week.