The Crowding Out Effect
The Crowding Out effect is a Monetarist criticism of expansionary fiscal policy. As seen in the multiplier effect, government spending will shift AD further than expected. However, Monetarists believe that because of this expansionary fiscal policy, the government will need to borrow money by selling government bonds. This leads to a rise in interest rates, i.e. from R1 to R3.
A rise in interest rates would discourage private investors from investing and private consumption may also decrease. So the result could be a rise from AD1 to AD2, instead of AD1 to AD3.