Secession is not illegal, there are some states, North Dakota, Wyoming, Nevada, Texas, Alaska and few others where they have a contract with the Federal Government and that contract is very specific on what the feds can and can not do and what they states can do if they are not happy. Texas for one has the contraction right to divide up in 5 areas and break away from the US, it is because the fact that it was a republic. I think PA can do the same thing, it is considered a commonwealth under the agreement with the feds.
BUT there are areas of the consitition that allow the states to do different things, one is a consitutional convention which if that would take place, it may change a lot of things. Repealing the 17th amendment and putting in a recall amendment would actually do a lot for the states and the people, it would shift control from the feds to the state and people, allowing the states to decide who and how to send senators to congress. By the way that is a very liberal thing, voting for senators. The recall amendment would unify all the recall laws under one consitutional amendment which would allow the states or people to decide if they want to recall a senator or congressmen without congresional involvment. It is the control we actually need to prevent the Levins and other life long do nothing senators.