What's the meaning of the bold sentence?

The quoted sentence says that "the plan" (that is the draft of the US Constitution) involves so many factors, affects so many people's situations, and will modify so many particular ways of doing things that when the plan is being discussed, that discussion will inevitably involve various topics that do not directly bear on whether the plan is a good one or not.

The Federalist is written in a rather formal style, even for its time, and styles have changed a bit since the late 18th century. It was generally thought to be well-written and highly intelligible at the time, and has often been quoted and analyzed as evidence of what the Constitution means, and as an explanation of its virtues. It was originally written as an act of political persuasion -- it was intended to persuade people to vote to ratify the the draft Constitution, particularly in the state of New York. It was first published in newspapers on a three-chapters a week schedule, and so was written in something of a hurry. Given that its quality of writing is in my view quite surprisingly high.

Let me analyze the sentence at hand.

The plan offered to our deliberations affects too many particular interests, innovates upon too many local institutions, not to involve in its discussion a variety of objects foreign to its merits, and of views, passions and prejudices little favorable to the discovery of the truth.

• "The plan offered to our deliberations" This is the subject of the sentence. It means the draft constitution.

• "affects too many particular interests," this is something the plan does. Note that at the time "interests" meant what we might now call "partisan views", things that were to the advantage or disadvantage of a person or a group. In particular, it most often meant things that were of financial advantage or disadvantage.

• "innovates upon too many local institutions" this is another thing that the pla does -- it makes changes ("innovates upon") in many different parts of the then existing government and society ("local institutions")

• "not to involve in its discussion" when the plan is discussed, one must consider a variety of things which might not at first seem relevant. This uses a negative form ("not") to imply that one might wish to avoid discussing these things, but cannot.

• "a variety of objects foreign to its merits" These are the things which will have to be discussed, an object of "involve" grammatically. "Objects foreign to its merits" means topics which do not bear directly on the advantages and flaws (the "merits") of the plan. "Foreign" here is used with the meaning of "separate from" or "apart from" not "coming from another country".

• "and of views, passions and prejudices little favorable to the discovery of the truth" This is another thing that must be "involved in the discussion" of the plan. This means that writers about the plan must consider and respond to things that people have said and thought that are more driven by emotion ("passions") than logic, and that can get in the way and confuse the subject, or even promote mistaken opinions ("little favorable to the discovery of the truth").

I don't see any cases of elision (omitted words) in the sentence. However, some words have somewhat changed senses since this was written, particularly "interests" and "passions".