News consumption via web portals is rapidly increasing in Korea. Web portals do not directly produce their news contents, but select, assign and post them, much like conventional media outlets. Then, do news web portals present similar political bias? To answer this question, this study conducted a big data analysis to objectively measure the political orientation of Korea’s major portals. Firstly, data was compiled on the expressions used by conservatives and progressives in official statements to address certain issues. Next, the relative frequency of these expressions and their significance was applied to measure the political orientation of portals. The results reveal that, web portals are not systematically biased in their selection of contents, but rather, swayed by public opinion. Specifically, profit in the web portal market is generated by the number of user clicks. As such, web portals are highly likely to select news contents that cater to the political leanings of their user base than the government or news providers. How does the political preference of users affect portals’ selection of news contents? Using data on consumer’s click rate and portal’s political orientation, an empirical analysis found that users favored portals that are closer to their political standing, and this in turn, determined the leanings of the respective portals.