In last night's debate, former Florida Governor Jeb Bush took an unexpected angle in his defense of the 43rd president of the United States. After Republican frontrunner Donald Trump mocked George W. Bush for ruining the Republican brand, his younger bother chastised the billionaire, saying that the last Bush in the White House "kept us safe" -- a line that spurred applause from the elite Reagan Presidential Library audience.

"You remember the rubble?" asked Bush. "You remember the firefighter with his arms around it? He sent a clear signal that the United States would be strong and fight Islamic terrorism, and he did keep us safe."

Trump largely let the line pass -- "I don't feel so safe" -- but on social media, it was a clanger. Jeb Bush had invoked what Esquire columnist Charlie Pierce calls "the great mulligan," the idea that a responsible discussion of the George W. Bush terror record starts on September 12, 2001. For a very long time, conservatives ruled out any criticism of Bush for his actions (or lack thereof) before 9/11 as bitter conspiracy-mongering. And on the stage last night, no Republican -- not even Sen. Rand Paul (R-Ky.) -- pushed back. That freed up Jeb Bush to emphasize his jarring argument that George W. Bush's speech from the rubble of the World Trade Center, which presaged the ongoing Afghanistan war, was exhibit A of how he "kept us safe."

On Twitter last night, all progressive critics of the Bushes could do was gawk.

The Bush tweet on Thursday sparked a fresh round of Web reaction:

And asked through a spokesman if Bush was right to say his brother "kept us safe," Paul, who is campaigning in Nevada today, declined to comment.

Iconic images from 9/11 and the aftermath

USA - YEAREND PICTURES 2001 - With the Brooklyn bridge in the foreground, a plane explodes after hitting the second tower of the World Trade Center as the other tower burns, in New York September 11, 2001. Both towers of the complex collapsed after hijacked planes hit them. REUTERS/Sara K. Schwittek ORG XMIT: POY33D ORG XMIT: M0112210347270387 (STR/REUTERS)