They are a candidate for re-election to an office that they already have. Obama will be running for President as the incumbent in the next Presidential Election.
They have name recoqnition, franking privlages, party support, money, support from their constituents. This more sums it up sorry it isn't more detialed.
The incumbent is the current holder of a political office. This term is usually used in reference to elections, in which races can often be defined as being between an incumbent and non-incumbent