Can I straighten a leaning elm tree with raised roots?

It all depends on the size of the tree. If it is a mature specimen, then attempts to straighten the tree will likely fail. If it is a younger tree, and the site is favorable for installing suitable anchors, then there may be a chance. However, steps should also be taken to ensure that the tree eventually stands on its own. This means getting the soil nutrition and irrigation regime right. It may also mean reconditioning the soil to provide a more favorable medium for aggressive root growth than mere fertilization.