"However brilliant the strategy may be, you cannot formulate a strategy and mandate that Congress implement it," Cohen, also a former Republican House member and senator from Maine, told a luncheon session with reporters.
Cohen said it has been his experience as a member of Congress that "the less they are involved at the beginning, the more they'll be involved at the end, in a less than positive way."
Cohen's comments came as Rumsfeld was slated to meet Thursday on Capitol Hill with members of Congress who will be influential in deciding whether his plans become reality.
"I view this as being an informal discussion,'' Senate Armed Services Chairman John Warner, R-Va., told the Associated Press.
The meeting will involve the top Republican and Democratic lawmakers on Warner's panel and the Senate Appropriations Defense Subcommittee. Separately, Rumsfeld was to meet with leaders of the corresponding committees on the House side, congressional aides said.
Cohen noted that he has not been involved in any of Rumsfeld's reviews and has not offered his advice to the new administration. But based on his experience, he said, "If you're going to have bold proposals, you have to bring Congress in early, rather than late."
That may run the risk of having the boldness of the plans scaled back early, but Cohen said "that's a risk you have to take" to avoid alienating the people who will be needed to implement the proposed changes.
He made the same observation about the military leadership, cautioning against considering the military "as a bloc that is uniform as an opposition to any change. I never found that."
But if they are not involved in shaping the proposed changes, "you have an almost natural coalition of [military] people you've excluded with people you've excluded on [Capitol] Hill. Then you have some very tall barriers you've got to overcome."
Cohen warned that the administration would have to send Congress a defense supplemental in the next month or so.
"If you don't have a supplemental fairly soon, then you'll see flying hours cut back, see training cut back... and you'll start to see the problems that we started to correct with retention and recruitment and morale," he said.