WASHINGTON – The Pentagon Tuesday offered a carefully calibrated plan for lifting the ban on gays serving openly in the military that would allow the military to keep President Barack Obama’s vow to end “don’t ask, don’t tell” while accommodating the substantial minority of troops who said repealing the ban would hurt their ability to fight wars.

In unveiling an eight-month-long study that included the largest survey of military opinion ever, Pentagon officials stressed that 70 percent of the more than 115,000 soldiers, sailors and Marines who responded said they thought there’d be no effect or positive effect from lifting the ban, which Congress codified 17 years ago during the administration of President Bill Clinton.

But that overwhelming support evaporated in combat units, according to the 256-page study, where 48 percent of Army troops and 58 percent of Marines thought lifting the ban would have negative consequences. More than a quarter of Army troops and more than a third of Marines said they’d consider leaving the military if the ban were lifted.

“In my view, the concerns of combat troops as expressed in the survey do not present an insurmountable barrier to a successful repeal of ‘don’t ask, don’t tell,’ ” Defense Secretary Robert Gates told reporters at a news conference to announce the report’s conclusions. “However, these findings lead me to conclude that an abundance of care and preparation is required if we are to avoid a disruptive – and potentially dangerous – impact on the performance of those serving at the tip of the spear in America’s wars.”

Among the steps the military would take to mitigate possible negative impact from a repeal, should Congress enact one, would be to limit its application to deployed units so that commanders’ attention can remain focused on combat, and not on the training and education that the Pentagon says will be needed for successfully incorporating openly gay people into the services.

The Pentagon also is proposing limiting housing benefits to married heterosexual couples only, while other military benefits, such as health insurance, would be made available to same-sex partners of service members.

It was unclear how the report will affect the debate in Congress on repealing “don’t ask, don’t tell,” which has been approved by the House but faces an uncertain future in the lame-duck Senate.

Gates urged Congress to repeal “don’t ask, don’t tell” during the lame-duck session. He said he fears that a recent court case, including an order in California that stopped the enforcement of the ban worldwide for eight days in October before an appeals court stayed it, could lead to abrupt change – “by far the most disruptive and damaging scenario I could imagine,” he said.

But he also said the military would need an unspecified amount of time after a congressional repeal to prepare and train the troops for the change, and that there may be a period when soldiers are being trained to accept gays even as the law still states they can’t serve.

Democrats who previously supported the repeal hailed the report’s findings. “Today’s report confirms that ending ‘don’t ask, don’t tell’ can be implemented in a manner consistent with maintaining the strong, cohesive military force we have today,” Sen. Carl Levin, D-Mich., the chairman of the Senate Armed Services Committee, said in a statement.