When a popular government ceases to execute its laws fairly, this can come only from the corruption of the republic and the state is already lost.

From The Spirit of the Laws by Montesquieu.  Chapter 3.  

On the principle of democracy

There need not be much integrity for a monarchical or despotic government to maintain or sustain itself. The force of the laws in the one and the prince’s ever-raised arm in the other can rule or contain the whole. But in a popular state there must be an additional spring, which is VIRTUE.

What I say is confirmed by the entire body of history and is quite in conformity with the nature of things. For it is clear that less virtue is needed in a monarchy, where the one who sees to the execution of the laws judges himself above the laws, than in a popular government, where the one who sees to the execution of the laws feels that he is subject to them himself and that he will bear their weight.

It is also clear that the monarch who ceases to see to the execution of the laws, through bad counsel or negligence, may easily repair the damage; he has only to change his counsel or correct his own negligence. But in a popular government when the laws have ceased to be executed, as this can come only from the corruption of the republic, the state is already lost.

It was a fine spectacle in the last century to see the impotent attempts of the English to establish democracy among themselves. As those who took part in public affairs had no virtue at all, as their ambition was excited by the success of the most audacious one and the spirit of one faction was repressed only by the spirit of another, the government was constantly changing; the people, stunned, sought democracy and found it nowhere. Finally, after much motion and many shocks and jolts, they had to come to rest on the very government that had been proscribed.

In a constitutional republic, such as ours, much of the effectiveness of our governance rests on the trust reposed by the citizens in the government.  A trust that is either bolstered or eroded by experience of the laws being fairly and equally administered.  A government of laws, not of men.  

District Attorneys are only one small cog in the vast machinery of our system of checks-and-balances; of federal, state and local authorities; of legislation, enforcement, and judgment.  Theirs is a small, but critical role.  Almost a lynchpin.  

When a small number of District Attorneys in select jurisdictions (primarily a couple dozen large cities) begin to explicitly and manifestly fail to enforce the law at all, or begin to administer the law selectively, there are two consequential outcomes.  

Tactically these District Attorney failures manifest as a desire to decarcerate prisoners, dispense with the bail process, discount police authority or testimony, preferring treatment over punishment, enforcing some laws and not others, beginning to enforce the law against some individuals or groups but not others, attempting to enforce local jurisdiction laws on individuals and enterprises not present in the jurisdiction, etc.  In these instances, the laws under the District Attorneys "have ceased to be executed."

The failures to enforce the law has two consequences, one immediate and direct and the other indirectly and in the future.

The immediate and direct consequence is that crime rises.  A lot.  Everywhere.  Both in kind and degree.  

Sometimes this is sufficient to drive local citizens to the polling booth and the driving out of the authors of these bad consequences.  But when that does not happen and the policies remain in place for more than a brief time?

Then you have the second consequence and it is strategically the worse consequence.  If the law is not enforced fairly and equally, then it becomes merely a utilitarian process.  Citizens begin to obey the law only to the extent that that they can see direct benefit to themselves.  There is no trust that the law will function fairly and for all and therefore there is an erosion in the moral authority of the law.  

Failure to uphold the law undermines it and as citizens begin to drift away into only a transactional adherence to the law, you end up with an obvious condition where there is no consent of the governed.

And any system of governance which does not have the consent of the governed will eventually, sooner or later, fail.