I'm quite sure that a contract would not be legally valid if I signed it while the other party was threatening me or someone I love with bodily or financial harm.
I know that the current negotiations going on in Wasington between the president and speaker of the house won't fall into the same catagory (i.e. they won't be invalidated because of undue pressure) but is seems disturbingly analogous. Actually, it's more like two spouses and the one with control of the checkbook is saying they will destroy both of their credit scores unless the other gives in to unrelated (or minimally related) demands.
But that's just the checks and balances that the US government was designed around, to make sure no individual or group would be able to amass too much power.
At the end of the day It was always known that it would take pretty serious negative results to force people to seek compromise despite their opposing interests.
If the president wants more power then he has to work with the legislative branch to gain it, and the threats flying back and forth are just part of that political process.
