Obama has threatened to veto the Keystone Bill should the new, Republican-dominated, Congress pass it. Obama has used the veto sparingly in his time in office, casting only 2 so far. However the key word here is ‘threat’. When discussing the powers of the President, the veto is a key power given by the Constitution. But a more astute student will look to discuss the difference between use of the veto and threatened use. In this instance, Obama is hoping to enact change by threatening the veto. He is capable of doing this as it is unlikely the required votes (2/3rds) would be found to overturn his veto, so either the bill needs to pass in modified form, or there is little point passing it at all.
Further, a President must not over use the veto, and be accused of tyranny. Indeed, even the threat of the veto must be used sparingly, otherwise Congress will begin to ignore such threats.
A useful example of more subtle checks and balances at play in US politics.