The best reason why direct democracy would not work well in the United States is that "<span>large groups of voters would be difficult to manage", since the population of Ancient Greece, where it was practiced, was much smaller than the current US population.</span>
An economic downturn.
http://factsanddetails.com/russia/History/sub9_1f/entry-4990.html
The Soviet Union turned a rural, agrarian society into one of the largest economic powerhouses in the world. The breakup greatly effected it.
Radical Republican, during and after the American Civil War, a member of the Republican Party committed to emancipation of the slaves and later to the equal treatment and enfranchisement of the freed blacks.
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The main competing arguments in the Webster-Hayne debate----which took place between Senator Daniel Webster of Massachusetts and Senator Robert Y. Hayne of South Carolina--were over protective tariffs in the United States, with one side arguing that states had a right to nullify laws they deemed unconstitutional, and the other argued that the Union needed to be protected at all costs.