"The statement was unclear. The Department of Defense has already decided to allow currently-serving transgender individuals to stay in the military, and many are serving honorably today. Any American who meets current medical and readiness standards should be allowed to continue serving," the Arizona Republican said in a statement. "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. 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."
McCain, the chairman of the Senate armed services committee, added that no decision is appropriate until the study is complete and reviewed by the secretary of defense, military leadership and Congress.
McCain's criticism comes one day after he dramatically returned to Washington following surgery for brain cancer. After voting to advance consideration of Republican health care plans, he delivered a well-received speech on the Senate floor calling for more bipartisanship in the chamber.
The Arizona Republican was joined by lawmakers on both sides of the aisle who pushed back against Trump's decision to reinstate the ban
barring transgender individuals from serving "in any capacity" in the US armed forces.
Trump's plan, which he announced via Twitter on Wednesday morning, reverses a policy initially approved by the Defense Department in 2016, which was still under final review, that would allow transgender individuals to openly serve in the military.
Some lawmakers denounced the move as discriminatory while others claimed potential economic benefits
of cutting Pentagon funding for transgender-related health care costs.
"You ought to treat everybody fairly and give everybody a chance to serve," Sen. Richard Shelby, R-Alabama, said on CNN's "Newsroom"
Wednesday, adding, "You have to remember our military force is a volunteer force."
House Minority Leader Nancy Pelosi in a statement that the policy change is a "cruel and arbitrary decision designed to humiliate transgender Americans who stepped forward to serve our country."
Rhode Island Sen. Jack Reed, the top Democrat on the armed services committee, noted the irony of the announcement's date.
"Today, on the anniversary of President Harry Truman's order desegregating the United States Armed Forces, President Trump is choosing to retreat in the march toward equality," Reed said in a statement.
Several other military veterans also spoke out against the decision.
"I served on active duty in the military, and I can tell you we don't care about gender orientation or identity or who you love, we just care if you can shoot straight and complete the mission," said Rep. Ted Lieu, D-California, on CNN's "Newsroom."
"This is actually hurting our military readiness, and I hope he reverses his decision."
Sen. Tammy Duckworth, an Illinois Democrat and retired Army lieutenant colonel, echoed similar sentiments in a statement.
"When my Black Hawk helicopter was shot down in Iraq, I didn't care if the American troops risking their lives to help save me were gay, straight, transgender or anything else. All that mattered was they didn't leave me behind," Duckworth said.
Iowa Republican Sen. Joni Ernst, who opposes the Pentagon's funding of gender reassignment surgeries, also denounced Trump's decision.
"As a veteran, Senator Ernst served alongside fellow service members from all different backgrounds and parts of the county. She believes what is most important is making sure service members can meet the physical training standards, and the willingness to defend our freedoms and way of life. While she believes taxpayers shouldn't cover the costs associated with a gender reassignment surgery, Americans who are qualified and can meet the standards to serve in the military should be afforded that opportunity," said Ernst spokeswoman Brooke Hougesen.
However, not all lawmakers have opposed the President's new policy.
"I'm pleased to hear that President Trump shares my readiness and cost concerns, and I'm glad the President will be changing this costly and damaging policy. Military service is a privilege, not a right. We must ensure all our precious defense dollars are used to strengthen our national defense. Now, we can focus on rebuilding our military and addressing the growing threats around the world," Missouri Rep. Vicky Hartzler, R-Missouri, told CNN.
Hartzler proposed an amendment
that was defeated last month that would have banned Pentagon funding for gender reassignment surgery and other transgender-related health care costs.
Here's how other lawmakers are reacting to the new policy:
Sen. Orrin Hatch, R-Utah:
"Yes" to a tweet asking "Hey, Utah lawmakers, do you stand with transgender Utahns?"
Sen. Elizabeth Warren, D-Massachusetts: "
The only thing -- only thing -- that matters when it comes to allowing military personnel to serve is whether or not they can handle the job," Warren tweeted
. "By attacking thousands of troops, @realDonaldTrump makes clear that he cares more about extreme ideology than military readiness...@realDonaldTrump can pretend this is a military decision, but it isn't. Banning troops on gender identity is shameful & makes us less safe."
Sen. Kristen Gillibrand, D-New York: "This morning, transgender service members put on their uniform and showed up for their military duties to be told by their commander in chief via Twitter that he doesn't want them in 'any capacity.' These service members are willing to die for their country, and this is an insult to their brave and honorable service. This new directive is harmful, misguided, and weakens -- not strengthens -- our military. I will introduce legislation and will fight to overturn this discriminatory decision," Gillibrand said in a statement.
Sen. Cory Booker, D-New Jersey:
"Transgender Americans in military are heroes like anyone else risking their lives to defend us. @POTUS is wrong," Booker tweeted
Sen. Kamala Harris, D-California:
"This is discriminatory, and un-American. To the transgender servicemembers who defend our nation, I stand with you," Harris tweeted
Sen. Chuck Schumer, D-New York:
"Transgender Americans are serving honorably in our military. We stand with these patriots," Schumer tweeted
Rep. Anthony Brown, D-Maryland:
"As a veteran and member of Congress I welcome transgender people into the military, and I will defend their right to serve our nation," Brown tweeted