Hi
Briefly, supremacy of EU law means that EU law takes precedence over national law. So states have limited there sovereignty by accepting EU law. The ECJ has ruled in a number of cases Van Gend en Loos (1963), Costa (1964).
So in answer to the question, the supremacy (or primacy) of EU law is already well established. The Reform Treaty (aka Lisbon treaty) which is being signed today will not state that EU law has primacy over national law, but it will include a declaration acknowledging the existing case law.
So omitting the primacy clause has more political significance than legal. The original Treaty of Rome (often referred to as The Treaty) has been amended by various other treaties but it has never explicitly stated the primacy of EU (previously EEC or EC) law. Instead, the ECJ established this. Whereas the Constitution Treaty (which failed) did explicitly state the primacy of EU law, for political reasons this has been left out of the Reform Treaty.
Supremacy is an important concept of EU law and most text book spend a whole chapter on it.
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