Sen. Barack Obama grabbed the early lead in the competition for delegates in Tuesday’s primaries.

Obama won at least 31 delegates in the North Carolina and Indiana primaries, according to an analysis of election returns by The Associated Press. Sen. Hillary Rodham Clinton won at least 20 delegates, with 139 still to be awarded.

In the overall race for the nomination, Obama leads with 1,776.5 delegates, including separately chosen party and elected officials known as superdelegates. Clinton has 1,628.

It will take 2,025 delegates to secure the Democratic nomination at the party’s national convention this summer.

Superdelegates are the party and elected officials who will automatically attend the national convention and can support whomever they choose, regardless of what happens in the primaries and caucuses.

The AP tracks the delegate races by calculating the number of national convention delegates won by candidates in each presidential primary or caucus, based on state and national party rules, and by interviewing unpledged delegates to obtain their preferences.

Most primaries and some caucuses are binding, meaning delegates won by the candidates are pledged to support that candidate at the national conventions this summer.

Political parties in some states, however, use multistep procedures to award national delegates. Typically, such states use local caucuses to elect delegates to state or congressional district conventions, where national delegates are selected. In these states, the AP uses the results from local caucuses to calculate the number of national delegates each candidate will win, if the candidate’s level of support at the caucus doesn’t change.