Congress cannot compel the President to testify.
Inherent power
Congress has several expressed powers. Some of these powers are the power to declare war, the power to naturalize citizens and the power to raise an army.
The elastic clause, which gives the Congress the power of executing "foregoing powers."
What power does congress have over a president?
Congress has been granted sovereign powers. These powers are the power to borrow money, the power to regulate Federal property, levying and collecting taxes, trade regulations, and military power.
Expressed powers are powers written down for Congress in the Constitution.
Enumerated powers
fiscal power, trade power, and military power
The congress had no power to tax unanimous decision for amendment, no separation of powers they could not get anything done.
The power of the purse.
The Power to have a military draft
the power to remove federal justices