Rahm Emanuel, the White House chief of staff, encapsulated President Obama’s goal long before the first of the First 100 Days: “Never let a serious crisis go to waste.”

That post-election formulation remains the fulcrum for the legislative battle only now starting in earnest. Over the Next 100 Days, the outcome may turn on how convincingly the White House preserves the link between Mr. Obama’s agenda and the painful recession he inherited.

Team Obama casts his initiatives on health care, energy, education and the auto and financial industries as responses to that crisis. His Republican adversaries call the recession merely an excuse for big-government ambitions that liberals have failed to achieve for decades.

Mr. Obama currently holds the upper hand, riding high in the polls while Republicans appear chaotic and hapless. But he is racing to capitalize for good reason. Political history, and some early signs this spring, suggest that time is not on his side.