Washington (CNN) -- Defense Secretary Robert Gates, Joint Chiefs of Staff Chairman Mike Mullen and the co-authors of a report backing a repeal of the military's "Don't Ask, Don't Tell" policy will testify Thursday before the Senate Armed Services Committee.
Both Gates and Mullen back a repeal of the law.
On Friday, the committee will hear from the top brass of the Army, Navy, Air Force and Marines.
Despite public opinion on the side of repeal supporters -- 58 percent approve, according to a Pew survey -- the heads of the four military branches have either directly opposed or been unenthusiastic about the policy change, at least until the Pentagon report was finished and released.
Marine Commandant James Amos has said that he opposes the change while the U.S. is fighting in Afghanistan because of its potential negative effect on unit cohesion. He will be joined on the second day of hearings by another Marine, the vice chairman of the Joint Chiefs, Marine Corps Gen. James Cartwright.
Gen. George Casey, the Army chief of staff, has said he is on board with Gates in considering the effect of the repeal. But committee members may remind him of something he told them earlier.
"I do have serious concerns about the impact of the repeal of the law on a force that's fully engaged in two wars and has been at war for eight and a half years," Casey said this year.
Once the Armed Services Committee's hearings are over, the spotlight will turn to Senate Majority Leader Harry Reid, D-Nevada. He will decide how the issue will move forward, whether to keep it part of the Defense Authorization Bill or whether to strip it off for a separate vote.
With just weeks left for this Congress, the calendar could be the biggest factor weighing on whether the law is repealed or upheld.
The House has already passed the repeal.
A long-awaited Pentagon review of the controversial "don't ask, don't tell" policy released this week said allowing openly gay or lesbian troops serve in the military would have little lasting impact on the U.S. armed forces.
Repealing the policy, however, would have "some limited and isolated disruption to unit cohesion and retention," the year-long study found, but the effects would not be long-lasting or widespread.
President Obama used the release of the report to urge the lame-duck Senate to follow the lead of the House of Representatives and pass a repeal of the Clinton-era law before the end of the year.
Gates also urged the Senate to act quickly, warning that the military doesn't want change imposed by "judicial fiat." Speaking at the Pentagon, he alluded to a recent string of court opinions calling the legal viability of the current policy into doubt.
There is, however, strong minority opposition to a change, particularly in the Marines and some combat arms specialist units, according to the chairs of the study, Defense Department General Counsel Jeh C. Johnson and Army Gen. Carter F. Ham.
As many as 40 to 60 percent of troops in those units were against changing the 17-year-old policy that lets gay and lesbian troops serve as long as their sexual orientation is secret.
Overall opposition throughout the military was about 30 percent -- roughly the same as it is in America as a whole, according to recent findings from CNN/Opinion Research Corp. and the Pew Forum.
More than nine out of 10 troops said their unit's ability to work with someone they thought was gay or lesbian was very good, good, or neither good nor bad.
Johnson told members of Congress on Tuesday that he thought "don't ask, don't tell" could be repealed even while the United States is at war, sources said.
The report also says that men and women removed from the military under the current "don't ask" rules should be allowed to return, without the reason for their dismissal being considered as part of their application.
The recommendations are based on surveys, focus groups and face-to-face meetings at bases around the world and even a carefully controlled effort to communicate anonymously with homosexuals serving in the military.
Social conservative activists quickly attacked the report.
"The surveys did not ask whether respondents support repeal of the current law," said Tony Perkins, head of the Family Research Council. "If most service members say that under a different policy, they would continue to attempt to do their job in a professional manner, that is only what we would expect. This does not mean that a new policy would not undermine the overall effectiveness of the force.
"If even a small percentage of our armed forces would choose not to re-enlist, or part of the public would choose not to serve in the first place, the impact on the military would be catastrophic," he argued, countering Johnson's claim.
CNN's Charley Keyes, Dana Bash, Chris Lawrence, Larry Shaughnessy, Alan Silverleib, Ed Hornick, Gabriella Schwarz and Paul Steinhauser contributed to this report.