Washington (CNN) Senate Democrats are gearing up to fight President Donald Trump's ban on transgender individuals serving in the military -- and they could have a key opportunity to do so as early as next week.

The Senate is tentatively planning to take up the National Defense Authorization Act, a major defense policy bill, when the chamber returns from its month-long August recess, multiple Senate aides tell CNN, though the schedule has not been finalized.

Democrats are working on crafting an amendment to the bill that could reverse Trump's directive barring transgender individuals from serving in the military, or at least protect transgender individuals currently serving, according to aides and advocates.

Trump signed the directive on Friday blocking new transgender recruits from joining the military and preventing the Pentagon from paying for medical treatment regimens for transgender individuals currently serving in the military.

The memorandum, which reversed the Obama administration's move in 2016 allowing transgender individuals to serve openly, also instructed Defense Secretary James Mattis to determine what to do with currently serving transgender service members based on "military effectiveness and lethality, budgetary constraints, and applicable law."

Mattis issued a statement Tuesday that the Pentagon will study the issue with a panel of experts and develop an implementation plan for Trump's directive by February 2018. Until the plan is completed, he said, transgender individuals currently serving will not be discharged based on gender identity.

The congressional debate over transgender service, shaping up to be the most contentious issue on the annual policy bill, is one of several avenues that opponents of barring transgender individuals from serving are exploring to fight Trump's memorandum. Transgender service members have also filed suit in federal court to challenge the directive.

Trump's initial tweet last month announcing he would prevent transgender individuals from serving in the military "in any capacity" -- which caught even the Joint Chiefs by surprise -- sparked a swift backlash from congressional Democrats. But the move also drew criticism from a number of Senate Republicans, including Senate Armed Services Committee Chairman John McCain of Arizona, Utah Sen. Orrin Hatch and Iowa Sen. Joni Ernst, an Iraq War veteran.

"I don't think we should be discriminating against anyone. Transgender people are people, and deserve the best we can do for them," Hatch said in a statement after Trump's July tweets.

Senate Democrats are hopeful they can get enough Republicans on board to pass an amendment blocking Trump's directive and preliminary talks are already underway about what kind of measure should be offered on the defense bill, according to Senate aides.

It's no sure thing, however, that any amendment targeting Trump's transgender directive would get to a vote. The defense policy bill, one of the few "must-pass" pieces of legislation remaining that has passed for more than 50 straight years, typically attracts hundreds of amendments and only a handful will get votes depending on negotiations between Republican and Democratic Senate leaders.

A spokesman for Senate Majority Leader Mitch McConnell did not respond to a request for comment. The White House also did not respond to a request for comment.

Sen. Kirsten Gillibrand, a New York Democrat, introduced an amendment to the defense policy bill last month to block Trump's ban and added a Republican co-sponsor in Sen. Susan Collins of Maine. Her measure would prevent the Trump administration from ending medical payments or discharging transgender service members.

"To all trans men and women serving bravely in the military: I plan to introduce legislation to fight back. We'll keep raising our voices," Gillibrand tweeted Friday responding to Trump's directive.

Gillibrand is expected to lead the Democratic effort on transgender service during Senate debate, but Democrats are considering several different legislative options to try to add to the defense bill, Senate aides and advocates working with lawmakers to draft a response told CNN.

Among the options under consideration is a measure that would codify into law the 2016 Obama administration directive on transgender service, preventing the executive branch from reversing it without Congress.

Carter's directive ended the previous ban on transgender individuals from serving openly and also instructed the Pentagon to decide how to allow new transgender recruits to join. Mattis had requested six more months to study the issue when Trump tweeted about barring transgender service members.

The House has already taken a vote on the issue -- before Trump announced his ban -- when 24 Republicans helped Democrats block an amendment to prevent the Pentagon from paying for the cost of transgender medical procedures.

The fate of the Senate effort opposing Trump's directive on the defense bill may lie with McCain, who is expected to return to Washington following his first round of cancer treatment to lead debate on the defense authorization bill.

In a statement last week, McCain said it would be "a step in the wrong direction" to force transgender individuals out of the military solely on the basis of gender identity. He added that he believes the Pentagon's current study should be completed before any decisions are made on new recruits.

A McCain spokeswoman declined to comment beyond McCain's statement on potential defense amendments.

Opponents of Trump's transgender service ban argue that it was a politically calculated move to discriminate against transgender individuals and that the President did not have any military basis the decision.

"It's not a surprise that there is considerable bipartisan concern about what's going on because the White House admitted that the original tweets were for an invalidly political purpose, which is why you had eight senators from the Republican Party speak out," said Aaron Belkin, director of the Palm Center, which conducts research on sexual minorities in the military.

Supporters say that the military should not be spending money on gender transition-related surgeries.

"Military service is a privilege, not a right," said Rep. Vicky Hartzler, a Missouri Republican who lobbied the White House to stop paying for transgender-related medical expenses before Trump announced the ban. "I'm pleased to see the president putting military readiness first and making sure our defense dollars are spent keeping us safe."