The case for universal preschool usually hinges on the effects found in the Perry Preschool and Abecedarian project. (See for instance, Heckman and Masterov "The Productivity Argument for Investing in Young Children".) Both find positive impacts at levels of spending per child that are higher than those seen in Head Start, for instance. Both these projects seem a little dated although I think it is important to continue tracking the adults -- the sample for Perry Preschool was drawn in the 60's and the Abecedarian sample was drawn in the 70's. In order to bolster the case for positive impacts of high quality childcare there should be funding for newer and bigger longitudinal studies. In addition, there should be a sample drawn perhaps every decade to continue to monitor the effects of quality child care.
It is also unfortunate that after almost 40 years none of the data have been made public (at least not on the websites cited above).
The motivation for this post was from Lane Kenworthy's excellent synthesis on findings of economic mobility.