In recent years, the fact-checking website PolitiFact has been paying close attention to this question, and its numbers are largely in line with what scholars find. Examining more than 500 promises President Obama made during his two presidential campaigns, PolitiFact finds that he has fully kept or reached some compromise on 70 percent of them. Similarly, Republican leaders made, by PolitiFact’s count, 53 promises before taking over Congress in 2010; 68 percent of these have been partially or fully kept.
This pattern isn’t unique to America. Scholars in Canada and Europe have examined the phenomenon and found their politicians to be, if anything, even more trustworthy. (The gap probably reflects added incentive — and increased opportunity — politicians have to carry out their policies in a parliamentary system where one party controls both the legislative and executive branches of government.) Across both time and borders, then, the data in this case is fairly clear. (Source)