I just ran across this recently-published paper in PNAS, by Rubin Enikolopov and colleagues, “Field Experiment Estimate of Electoral Fraud in Russian Parliamentary Elections.” Here is the abstract:

Electoral fraud is a widespread phenomenon, especially outside the developed world. Despite abundant qualitative and anecdotal evidence on its existence from around the world, there is very limited quantitative evidence on the extent of electoral fraud. We exploit random assignment of independent observers to 156 of 3,164 polling stations in the city of Moscow to estimate the effect of electoral fraud on the outcome of the Russian parliamentary elections held on December 4, 2011. We estimate the actual share of votes for the incumbent United Russia party to be at least 11 percentage points lower than the official count (36% instead of 47%). Our results suggest that the extent of the fraud was sufficient to have had a substantial impact on the outcome of the elections; they also confirm that the presence of observers is an important factor in ensuring the integrity of the procedure.