A pretty comprehensive quantitative study of the effectiveness of democracy support (Finkel et al 2007& 2008) found that, when controlling for a wide variety of other factors, higher levels of assistance to democratisation are, on average, associated with movement to higher levels of democracy. These results provide the clearest evidence to date that democracy assistance contributes toward achieving its desired goals.
But it also bemoans that some countries receiving very substantial US democratisation funding such as “Egypt and post-Soviet Russia” have shown disappointing progress. That was 2008.