But in many cases, lawmakers gave up on the administration-led operation weeks ago, turning instead to their own connections to foreign officials, private organizations and individuals navigating private charter flights in and out of Afghanistan.

Concerned donors have given millions to an effort, called Operation Flyaway and led by the firm Raven Advisory, to assist with evacuations, compiling a list of more than 4,000 Afghans whom Americans were seeking to help. Veterans’ groups have sprung into action in volunteer efforts to organize what many call a digital Dunkirk. Some have taken it upon themselves to try to rescue Afghans they know personally from their time overseas.

Together with congressional offices, an informal support network has arisen to fill the gaps.

“It’s inspiring,” said Mr. Waltz, a former Green Beret who served in Afghanistan. “But at the same time, I think it’s a real testament to how badly the administration has dropped the ball.”