Why did President Johnson withdraw from the 1968 presidential race?A) He was suffering from ill health // B) He was shocked by U.S. losses in...

I concur with Pohnpei, the best answer in this case is B.
The president was shocked by U.S. losses in the Tet Offensive. The war in Vietnam
brought about a myriad of challenges to the president. These challenges caused the rapid
drop in popularity for the presidency among the citizens. During his campaigns, the
president had assured the public that he would not expand the United States involvement
in Vietnam. He however, did not adhere to his plan, and this resulted in massive losses
for the U.S. in Vietnam. The country suffered increasing casualties and negative
financial effects caused by the expansion efforts in
Vietnam.

The defeat during the Tet offensive offered more
opportunities to his opponents to continue challenging the United States involvement in
Vietnam. The general population was sharply divided on the issue, with some of the most
influential politicians, such as Robert F. Kennedy, increasingly campaigning against the
war in Vietnam.