Chair Janet Yellen testifies before Congress this week with the Federal Reserve facing its gravest political threat since the drafters of the Dodd-Frank act tried to strip it of its supervisory powers.

The Fed is being pressured from the left and the right. U.S. Senator Elizabeth Warren of Massachusetts and other Democrats have blasted the central bank for being too cozy with the banks it oversees. Republicans, including potential 2016 presidential contender U.S. Senator Rand Paul of Kentucky, have focused on its aggressive monetary policy.