Several people close to President Biden said on Thursday that they believe he has begun to accept the idea that he may not be able to win in November and may have to drop out of the race, bowing to the growing demands of many anxious members of his party. One of the people close to him warned that the president had not yet made up his mind to leave the race after three weeks of insisting that almost nothing would drive him out. But another said that “reality is setting in,” and that it would...