The declarations removed America's political barriers

Wasn't the existing German-American conflict enough to justify a declaration after Pearl Harbor?

It supplied possible grounds, but Roosevelt still needed congressional support; the Axis declarations removed much of the opportunity to delay American entry.

A presidential request to open war against Germany and Italy would have needed support through both houses of Congress. The Axis declarations neutralized isolationist and pro-Axis delaying tactics at once. Without them, Roosevelt would have faced a slower democratic process even amid an undeclared Atlantic conflict. American bombers, troops, and a Europe-first strategy might consequently have been delayed for months or years while the declared Pacific war absorbed attention and resources.