This paper investigates the positioning in political news coverage of the big four US newscasts, ABC News, CBS News, FOX News and NBC News, as well as how they alter their positions when president Barack Obama took over from George W. Bush. Our empirical analysis is based on the tonality of their political coverage using more than 815,000 news items on Democrats and Republicans from 2001 through 2012. Using the Political Coverage Index (PCI), we find evidence for partisan and biased news coverage across all of the newscasts, with a general tendency of ABC News, CBS News, and NBC News to be slightly more critical of the Republicans, and of FOX News with a clear tendency to be more critical of Democrats. Moreover, and even more interesting, two of the newscasts – CBS News and NBC News – changed their political coverage such that it became more conservative when Democrat Barack Obama became president. This effect becomes even more pronounced when observations from election campaign periods are dropped. We interpret these results as evidence for a watchdog-like anti-government bias on the part of the observed newscasts. In contrast, FOX News remains Democrat-critical independent of who is in office and can be seen as partisan from this perspective. The results are confirmed when controlling for differences in reporting rates between the parties and the administrations using interaction terms.