The Constitution is silent on how presidential candidates should be nominated. "The Founding Fathers assumed that states would assign their electoral votes to individuals with strong local and national reputations," says Richard Baker, the US Senate historian.
The rise of party politics changed that calculation. Early in the 19th century, party congressional delegations, by default, selected the nominees. The chaotic election of 1824, which gave rise to the modern Democratic Party, also led to the creation of party nominating conventions outside Congress.
But the role of lawmakers and other party professionals in presidential nominating conventions has shifted over time. The nomination of Vice President Hubert Humphrey, who had not won a single Democratic primary, in 1968 set off a backlash among party activists. The primary and caucus system they set up, including rule changes as to who could attend the Democratic Party's national convention, marginalized the party professionals, including members of Congress.
Party leaders say it contributed to a landslide loss for Democratic nominee George McGovern in 1972. "One of the major unintended consequences of reform was that members of Congress and other elected officials of the party weren't going to convention anymore," says Rep. David Price (D) of North Carolina, who helped draft party reforms creating a system of unpledged degates, now known as superdelegates, in the 1970s.