It's not just that this is a clear rebuke of his confident prediction 2½ weeks ago ...
"I believe we can get it done," he said. "I'll see you at the bill signing."
... It's that the defeat was sealed by his own party. After personally lobbying many Republicans, more than three dozen defected.
Immigration was Mr. Bush's signature domestic issue, and he used whatever remaining political capital he had left. Remember those hot and sweaty trips to the Southwest so he could patrol the border?
That's exactly why historians will look back on this as a defining day in the Bush presidency, CBS News chief White House correspondent Jim Axelrod reports.
And it's not just immigration. When the White House claimed executive privilege today toin the U.S. Attorney firings investigation, a top Republican complained the White House was just continuing to protect the president's embattled attorney general.
"And while the investigation is lagging, Attorney General Gonzalez continues to serve. But as long as he continues to serve, the department is in disarray," Sen. Arlen Spector, R-Pa., said.
Add to that this week's high-profile Republican deflections about the war, which sent national security adviser Steven Hadley to Capitol Hill to see if he could contain the damage.
"When people in your own party turn on you when you're a president is when your policies crumble," says Brinkley.
One GOP insider told Axelrod today: "When it comes to the president's influence, it's 'how low can you go.' If the president's approval ratings were just at traditional lows, things might be different. They're not. They're at historic lows. So now it's every man for himself.