executive

[NewHope]

The executive = the President of the US?

it is scarcely the principal, object of my jealousy = The object of my jealousy is scarcely the principal?

Context:

Jefferson sounded a similar warning when he said: "The executive, in our government is not the sole, it is scarcely the principal, object of my jealousy. The tyranny of the legislature is the most formidable dread at present and will be for many years.

[Francois]

The executive = the President of the US?

It refers to the executive branch of power (in contrast with legislature here). The President embodies this group.

it is scarcely the principal, object of my jealousy = The object of my jealousy is scarcely the principal?

No, it should read: ...is not the sole, and even not the principal, object of my jealousy.
=> It is not the sole object of my jealousy (there are others)
=> This is not the principal object of my jealousy (there are others that bother me more).

FRC