Divisiveness.

John Adams said:There is nothing which I dread so much as a division of  the republic into two great parties, each arranged under its leader, and  concerting measures in opposition to each other. This, in my humble  apprehension, is to be dreaded as the greatest political evil under our  Constitution.

George Washington agreed, saying in his farewell presidential speech:

The alternate domination of one faction over another, sharpened by the  spirit of revenge, natural to party dissension, which in different ages  and countries has perpetrated the most horrid enormities, is itself a  frightful despotism. But this leads at length to a more formal and  permanent despotism. The disorders and miseries, which result, gradually  incline the minds of men to seek security and repose in the absolute  power of an individual; and sooner or later the chief of some prevailing  faction, more able or more fortunate than his competitors, turns this  disposition to the purposes of his own elevation, on the ruins of Public  Liberty.

Without looking forward to an extremity of this kind, (which  nevertheless ought not to be entirely out of sight,) the common and  continual mischiefs of the spirit of party are sufficient to make it the  interest and duty of a wise people to discourage and restrain it.

It serves always to distract the Public  Councils, and enfeeble the Public Administration. It agitates the  Community with ill-founded jealousies and false alarms; kindles the  animosity of one part against another, foments occasionally riot and  insurrection. It opens the door to foreign influence and corruption,  which find a facilitated access to the government itself through the  channels of party passions. Thus the policy and the will of one country  are subjected to the policy and will of another.

There is an opinion, that parties in free  countries are useful checks upon the administration of the Government,  and serve to keep alive the spirit of Liberty. This within certain  limits is probably true; and in Governments of a Monarchical cast,  Patriotism may look with indulgence, if not with favor, upon the spirit  of party. But in those of the popular character, in Governments purely  elective, it is a spirit not to be encouraged. From their natural  tendency, it is certain there will always be enough of that spirit for  every salutary purpose. And, there being constant danger of excess, the  effort ought to be, by force of public opinion, to mitigate and assuage  it. A fire not to be quenched, it demands a uniform vigilance to prevent  its bursting into a flame, lest, instead of warming, it should consume.

“All men having power ought to be distrusted to a certain degree.”

“Knowledge will forever govern ignorance, and a people who mean to be
their own governors must arm themselves with the power knowledge gives.
A popular government without popular information, or the means of
acquiring it, is but a prologue to a farce or a tragedy or perhaps both.”

“You must first enable the government to control the governed; and in the next place, oblige it to control itself.”

"No nation could preserve its freedom in the midst of continual warfare.”

―  James Madison, in Letters and The Federalist Papers