Much less opportunity is not quite right. Most people who get elected have their price. What you need to do instead is make the bribery illegal. Of course, it is not straightforward to do this when the people making the laws have been bought or promised cushy, high-paying jobs when their term limit is up. As it is shockingly cost-effective, the companies will keep doing it. Long story short, the defense you need is, therefore, the dictatorship of the proletariat, and that defense can only be achieved by a revolution.