The Defense Department has issued a memorandum to a group of former military commanders and civilian analysts that offers a direct challenge to the criticisms made by retired generals about Defense Secretary Donald H. Rumsfeld.

The one-page memorandum was sent by e-mail on Friday to the group, which includes several retired generals who appear regularly on television, and came as the Bush administration stepped up its own defense of Mr. Rumsfeld. On the political front, Republican strategists voiced rising anxiety on Saturday that without a major change in the course of the Iraq war, Republican candidates would suffer dearly in the November elections.