I was taught that the U.S. Consitution provides a system of checks and balances for the three branches of government. Congress can have its laws repealed by the Judiciary, and the Executive can be denied funding by the Congress, just for a couple of top-of-my-head examples.
The system may still be in the text, but the branches of government have decided they can interpret the Constitution as they desire. The system depends on a somewhat adversarial relationship between the branches. If all three are in basic agreement, Federal power is not checked and the whole structure goes out of balance.
And it really doesn’t matter that different political parties and factions have more weight in different branches. They’ll surely find things to argue about, but in a broader view all the big factions have a vested interest in bigger government. Constitutional limits are not vigorously enforced.