The president has called the negotiations with Turkey after the fallout from his Syria withdrawal a "big success."
"Everyone can see the atrocities that are happening right now in Syria with the Kurds," McCain argued on ABC's "The View."
"This is a complete and utter national security disaster in the making and I hope President Trump will adjust his thinking," the Republican senator tweeted.
Following Trump's order to withdraw U.S. forces, the Kurds have allied with the Iranian and Russian-backed government of Syrian President Bashar al-Assad.
Trump gave Turkey the green light to invade northeastern Syria after a phone call with Turkish President Erdoğan last Sunday.
"We've won. We've beat ISIS and we've beat them badly and decisively," the president told reporters last week.
"I don't understand how a president that claims he's the toughest President ever would do such a weak move like this," Representative Adam Kinzinger said.
"This is heart-rending for anybody who has shed blood, who has deployed forward," former NATO Supreme Allied Commander James Stavridis said.
The president "recognized that sending American troops to Syria is not in the best national security interest of the United States," Andrew Napolitano said.
As Turkey launches offensive operations into northeastern Syria against once American-backed Syrian militias, the danger facing U.S. forces in the region have heightened amid reports that U.S. operations against the Islamic State militant group in Syria have halted.
"The fight against Daesh [ISIS] is not over and continues alongside the [Kurdish-led] Syrian Democratic Forces," Prime Minister Edouard Philippe said.
The Syrian military and the Syrian Democratic Forces are both advancing rapidly against the last of ISIS. What happens when the jihadis are gone?
U.S. allies in Iraq are also supporting the operation.
Washington has said that both sides should focus on fighting ISIS.
If the rebels take Jarablus, it would preclude an assault by Kurdish forces who hope to expand their reach.
Al-Shadadi is a key logistics hub for the group.