Second, Congress should expand access to oil shale -- a domestic resource that could produce the equivalent of more than a century's worth of imports at current levels. Last year, however, Democratic leaders slipped a provision blocking oil shale leasing on Federal lands into an omnibus spending bill. They should lift that ban immediately
这段Democratic leaders slipped a provision blocking oil shale leasing on Federal lands into an omnibus spending bill. 这句话怎么理解？

这句话你断的不对，leasing on不是连在一起的，应该这样断：
Democratic leaders slipped a provision [(blocking oil shale leasing) on Federal lands] into an omnibus spending bill.
现在理解了吧？全句这样翻译：
民主党领导人推动了[限制出租(联邦土地上)的石油页岩]的法案成为综合性的开支议案。

有些词可能翻的不太准确，但句子大体结构意思就是这样的。