What general political trend coincided with a significant increase in private and corporate spending in political campaigns at the turn of the twenty-first century?

The answer is " A withdrawal of people from the public sphere".

To put it in an unexpected way, when the peak of the 'political' is accomplished, which is the 'choice'; we are confronted at the same time with a depoliticization procedure that restrains the political subject. This sort of subduing or compelling can take an assortment of structures and more often than not leads toward an expanding of technologization and bureaucratization that supplant the 'political' and makes the circle of governmental issues removed and unattainable by overall population. Thus the general population pull back from open circle and disavow even their fundamental political rights as well as commitments, for example, voting investment.