President Donald Trump’s announcement Wednesday morning on Twitter that he will bar transgender people from serving in the military brings to a boil a previously simmering congressional debate.

Critics of Trump’s proposal have already vowed to fight back hard, and the battle will be joined promptly. It will start in the next 24 hours or so during House debate on security spending legislation.

The debate will rage on when the Senate takes up its fiscal 2018 defense authorization bill, probably starting next week. Trump’s critics got a major boost Wednesday when Arizona Republican John McCain, chairman of Senate Armed Services, who will manage the authorization bill on the floor, issued a statement blasting Trump’s proposal.

“There is no reason to force service members who are able to fight, train, and deploy to leave the military — regardless of their gender identity,” McCain said. “We should all be guided by the principle that any American who wants to serve our country and is able to meet the standards should have the opportunity to do so — and should be treated as the patriots they are.”

Assuming Trump turns his tweet into an order, the question quickly becomes: What are the odds Congress or the courts could overturn Trump’s move? McCain’s opposition instantly ups the odds for proponents of transgender people serving in the armed forces.

“After consultation with my Generals and military experts, please be advised that the United States Government will not accept or allow transgender individuals to serve in any capacity in the U.S. Military,” Trump tweeted. “Our military must be focused on decisive and overwhelming victory and cannot be burdened with the tremendous medical costs and disruption that transgender in the military would entail. Thank you.”

Swift reaction

Besides McCain, Republicans, with a few exceptions, were notably silent about Trump’s tweet after it was announced. By contrast, Democrats’ outrage was swift and severe.

Sen. Jack Reed of Rhode Island, the top Democrat on Armed Services and a senator who works closely with McCain on many issues, released a statement calling Trump’s announcement “a divisive political move.”

“In the land of the free and the home of the brave, every American who is brave enough to serve their country should be free to do so,” Reed said.

Sen. Kirsten Gillibrand, a New York Democrat who also serves on Armed Services, promised to file a measure to overturn a Trump order to bar transgender service personnel — and she will almost surely not be alone in drafting a legislative counterpunch.

“This new directive is harmful, misguided, and weakens — not strengthens — our military,” Gillibrand said in a statement.

In the House, Democrats were just as angry. Minority Leader Nancy Pelosi of California called Trump’s message part of a “vile and hateful agenda.”

Rep. Adam Smith of Washington, the top Democrat on House Armed Services, issued a statement calling Trump’s proposal “an unwarranted and disgraceful attack” on transgender people in the military, whom he numbered in the thousands.

Smith wondered what happened to Defense Secretary James Mattis’ ongoing reappraisal of the Obama administration’s decision last year to permit transgender troops to serve. McCain made a similar point.

Smith also said he was miffed and quoted Trump saying on Jan. 31 that he is “respectful and supportive of LGBTQ rights.”

Rep. Adam B. Schiff called Trump’s position “ugly and wrong,” and fellow California Democratic Rep. Ted Lieu said Wednesday was “a sad day for America.”

Floor debates

The House Rules Committee is expected to decide later Wednesday whether to permit floor consideration of any of a handful of amendments to the security spending bill that pertain to transgender people in the military. The amendments, filed by members on both sides of the debate, mostly address whether or not medical services for transgender service members will be paid for by Pentagon funds.

If the president is able to bar transgender troops, though, the question of paying for their medical services will be moot. Regardless, if an amendment on the subject is made in order, it will provide a forum for lawmakers to vent on the issue.

When the House debated its defense authorization bill earlier this month, it narrowly defeated, 209-214, an amendment by Missouri Republican Vicky Hartzler that would have barred funding for such medical treatment.

At a July 24 Rules meeting on the security spending bill, Texas Republican Louie Gohmert said he believed the House would adopt Hartzler’s proposal if the chamber took a new vote.

The American Civil Liberties Union, in a statement, hinted it may sue to block Trump from implementing the order. ACLU lawyer Joshua Block said in the statement that the group’s members are “examining all of our options on how to fight this.”

The Senate’s coming authorization debate is all but certain to feature heated exchanges on this subject, particularly given that the Republican and Democratic managers of the bill — McCain and Reed, respectively — are firmly against Trump on this matter.

It’s a safe bet that Democrats would try to block consideration of the bill if they do not have an opportunity to offer amendments to address Trump’s action. It’s also now clear that they have an ally in McCain, making it less likely they will need to resort to that tactic.