WASHINGTON — The Obama administration has initiated a last-ditch diplomatic campaign to avert a confrontation this month over a plan by Palestinians to seek recognition as a state at the United Nations, but it may already be too late, according to senior American officials and foreign diplomats. The administration has circulated a proposal for renewed peace talks with the Israelis in the hopes of persuading the Palestinian president, Mahmoud Abbas, to abandon the bid for recognition at the annual gathering of world leaders at the United Nations General Assembly beginning Sept. 20. The administration has made it clear to Mr....