Nancy Pelosi is Wrong–Congress is not co-equal to the president, the framers intended it to be superior.

House Speaker Nancy Pelosi got it wrong in recently declaring Congress as co-equal to the
president.  The reality is that the Constitution and its framers endorsed the concept of legislative  supremacy where Congress is supposed to be the dominate branch of government.   This supremacy  is the key to checking abuses of presidential power.
When Speaker Pelosi initially invited President Trump to deliver the State of the Union speech to Congress on January 29, she wrote: "The Constitution established the legislative, executive and judicial branches as co-equal branches of government, to be a check and balance on each other."
As the framers wrote the Constitution, they did so with the belief in legislative supremacy and the idea that Congress would be the most powerful branch of the government.  Separation of powers and checks and balances were critical features to the new constitution, but  branch co-equality was not.
Consider first the intellectual and historical circumstances surrounding the writing of the Constitution.  The American revolutionary war, as described in the Declaration of Independence, was largely about fear of the British monarchical executive power of King George III.  Much of the Declaration reads as a bill of particulars against the abuses of the king.  In arguing against monarchical powers, the framers were part of a long intellectual and historical tradition going back to seventeenth century England, if not to the Magna Charta,  arguing against the absolute power of kings and queens and in favor of a parliament. 
The British civil wars and the Glorious Revolution were about that.  The American founders were heavily influenced by British political philosopher John Locke who argued precisely in favor of parliamentary supremacy, against monarchical absolutism, and in favor of separation of powers.  Similarly, French political philosopher Baron de Montesquieu, who also  influenced the framers of the Constitution, too argued for separation of powers, but never asserted that the three branches of government would be co-equal.  Instead, he saw legislative authority as supreme and needing checks on it.
At the 1787 constitutional convention several of the framers argued the case that the legislature (Congress) needed to be checked because it was so powerful and, almost quoting Montesquieu verbatim,  could threaten the executive branch.  In part, the fear of Congress’ power was in part the basis of bicameralism (dividing the legislature in two to check its power) and giving the president a veto.
Further proof of legislative supremacy can be found in the text of the Constitution.  Article I which vests legislative power in Congress contains far more enumerated and implied powers than does Article II which vests executive powers in the president.  If a simple listing of the number of powers assigned to each branch means anything in terms of how to assess the relative powers of the two branches, by far Congress was more powerful.
But turn also to the Federalist Papers, considered by many including numerous Supreme Court Justices the definitive statement by Alexander Hamilton, James Madison, and John Jay regarding what the Constitution was supposed to mean.  In Federalist 39 Madison states:
But if the government be national with regard to the OPERATION of its powers, it changes its aspect again when we contemplate it in relation to the EXTENT of its powers. The idea of a national government involves in it, not only an authority over the individual citizens, but an indefinite supremacy over all persons and things, so far as they are objects of lawful government. Among a people consolidated into one nation, this supremacy is completely vested in the national legislature.

Additionally, in Federalist 48 Madison declares:
The legislative department derives a superiority in our governments from other circumstances. Its constitutional powers being at once more extensive, and less susceptible of precise limits, it can, with the greater facility, mask, under complicated and indirect measures, the encroachments which it makes on the co-ordinate departments. It is not unfrequently a question of real nicety in legislative bodies, whether the operation of a particular measure will, or will not, extend beyond the legislative sphere.

History and framers’ intent do not support the idea of co-equality.  Historian Arthur Schlesinger, Jr.  in his 1973 The Imperial Presidency made the same point, as did political scientist David Siemer in his 2018 The Myth of Coequal Branches: Restoring the Constitution’s Separation of Functions.   
What has happened over time is that both as a result of executive branch usurpation and congressional  acquiescence, the president has become too powerful.  Post-Watergate, there were efforts to correct this balance, but they lapsed. And as we have seen in the last few administrations, presidents continue to exercise too much power through executive orders and other actions,  and they need to be checked.  Congress reasserting itself as the dominant branch of the national government is one way to do that.