Saving face.
The shenanigans with Murkowski and threatening aid to Alaska is not a new tactic. It has been done forever. However, it is usually carried out quietly. You do this for us here and this will happen for you. It may be as simple as slipping an amendment in a huge bill with the understanding it would not be opposed.
Trump has to make sure everyone knows about his threats. He wants to bully and make sure people knuckle under to him. This was probably doubly so with Collins and Murkowski dealing with women drives him nuts.
By making this demand public, he set up a direct clash without understanding the repercussions. Nobody wants to be seen as being bullied and humiliated. 'Face' is an oriental concept but various uses of it can be found everywhere.
Lyndon Johnson was a master at being the invisible hand. He knew every Senator and who they represented and where their interests were. The pressure could come in many ways. Johnson could jerk somebody up in public. However, he preferred other methods.
This is why the conflict with North Korea is so dangerous. It is being set up as a direct confrontation with KJU. If anybody knows he cannot blink, it's him. His power depends on ruthlessness and fear. We have nobody who can step in and try to build a golden bridge to de escalate the tension. It will not end well.
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lbj's secret weapon was eating raw onions before giving senators "the johnson treatment"
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In 1994, Jimmy Carter went to North Korea on behalf of the Clinton administration and met with Kim Il-sung (Kim Jong-il's father). Because of his nuclear engineering background, he knew what he was talking about and he also knew what Kim Il-sung wanted in return - respect. That's what it took. His negotiations were successful and the relationship continued with SoS Madeline Albright and Kim Jong-il, who was willing to continue the agreement made by his father.
When George W Bush* became POTUS, Kim Jong-il asked if he could contunue dealing with Madeline Albright. The agreement fell apart in 2002 when Bush* included North Korea in his "Axis of Evil." That's all it took. And things have gotten progressively worse ever since. Sometimes diplomacy works - but it takes a diplomat to pull it off. 
We know that Kim Jong-un is not the most stable of characters and - from what I've learned from President Carter - bullying and threats are the worst tactic to take with him. But we know that Trump has no appreciation of diplomacy, let alone understanding or appreciation of other cultures, so having him as "the decider" just now couldn't be worse. And it's not like he's surrounded himself with anyone who has the skills or experience to deal with these delicate situations, either. He expects his real estate son-in-law to achieve Middle East peace... 
