Robert Epstein, a senior research psychologist at the American Institute for Behavioral Research and Technology, and colleague Ronald E. Robertson recently conducted an experiment in which they conclude that Google (goog) has the power to rig the 2016 presidential election. They call it the Search Engine Manipulation Effect (SEME).
Through five experiments in two countries, they found that biased rankings in search results can shift the opinions of undecided voters by 20% or more, sometimes even reaching as high as 80% in some demographic groups. If Google tweaks its algorithm to show more positive search results for a candidate, the researchers say, the searcher may form a more positive opinion of him or her.
Considering the fact that most presidential elections are won by small margins, Epstein believes that biased Google search rankings could potentially decide the outcome of an election.
In an op-ed the researcher wrote for Politico, he says that this kind of power in the hands of one organization is dangerous:
Because SEME is virtually invisible as a form of social influence, because the effect is so large and because there are currently no specific regulations anywhere in the world that would prevent Google from using and abusing this technique, we believe SEME is a serious threat to the democratic system of government.
Google responded to the research with the following:
Providing relevant answers has been the cornerstone of Google’s approach to search from the very beginning. It would undermine the people’s trust in our results and company if we were to change course.
Earlier this year, the European Commission formally accused Google of using its dominant market share in web search to steer users away from competing products and services and toward its own.
Subscribe to Data Sheet, Fortune's daily newsletter on the business of technology.