Wikipedia: Brokered Convention

"Before the era of presidential primaries, conventions were routinely brokered. Adlai Stevenson in 1952 for the Democratic Party and Thomas Dewey in 1948 for the Republican Party were the last two candidates selected through a brokered convention.

Since then, there have been many years where brokered conventions were projected, but did not come to pass."

http://en.wikipedia.org/wiki/Brokered_convention
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A brokered convention refers to a situation in United States politics where there are not enough delegates obtained during the presidential primary and caucus process for a single candidate to obtain a majority for the presidential nominating convention. Since no candidates receive enough votes on the first ballot to win the nomination, the convention is brokered through political horse-trading and multiple ballots.

Before the era of presidential primaries, conventions were routinely brokered. Adlai Stevenson in 1952 for the Democratic Party and Thomas Dewey in 1948 for the Republican Party were the last two candidates selected through a brokered convention.

Since then, there have been many years where brokered conventions were projected, but did not come to pass. In 1988, a brokered convention was predicted for the Democrats since multiple candidates won the Super Tuesday primaries that year.[1]

Several factors encourage decision in the primary process. First, candidates tend to get momentum as they go through the process, due to the bandwagon effect. Thus, one or two candidates will be portrayed by the media to voters as the front runner due to their placement in the first primaries and caucuses, and as also-ran candidates drop out, their supporters will tend to vote for the leaders. [2] Theorists have identified two types of political momentum, piecemeal and all-at-once, with different impacts on front-runners and those right behind them. [3] Secondly, political parties wish to avoid the negative publicity from a brokered convention, as a candidate nominated from the brokered convention will be seen as weak and must climb additional hurdles to gain election.