"At some point it'll be clear that there is a frontrunner, and at that point we would hope that, for the greater good, for the country, that we can all rally as early as possible behind one person," Pelosi told ABC Television on Tuesday.
Pelosi said it would be harmful for so-called superdelegates - senior Democratic officials who cast deciding votes if the neither candidate can secure an outright victory - to go against the choice of the party.
"I do think that as it evolves, one of them is going to have to realise that the numbers, whether its Senator Obama, and he would step aside, or whether its Senator Clinton," Pelosi added.
Clinton and Obama are involved in a close race to win enough delegates to secure the Democratic presidential nomination.
Obama currently leads by 162 pledged delegates, 1415-1253, according to an NBC news count. A total of 2,025 delegates are required to secure the nomination outright.
Pelosi, however, said she was not suggesting Clinton withdraw from the race.
Clinton holds a double-digit lead in recent polls of voters in Pennsylvania, which holds the next major primary vote on April 22.
Pelosi also spoke out on China and called on George Bush, the US president, to consider boycotting the opening ceremony of the Beijing Olympics in protest over China's crackdown on demonstrators in Tibet.
"I think boycotting the opening ceremony, which really gives respect to the Chinese government, is something that should be kept on the table."
Her remarks come as China launched the longest and one of the most controversial torch relays ever on Tuesday amid tight security and great secrecy over the exact route.
Security has been very tight after anti-government rallies last month in Tibet and the exact route of the relay - set to include an ascent of Mount Everest in the troubled region - will be kept secret to prevent any potential protesters from getting close.
Pro-Tibet protesters around the world have been demanding that the IOC and world leaders boycott the games.
Nicolas Sarkozy, the French president, was the first European leader to suggest a boycott of the opening ceremony to protest against China's handling of the unrest in Tibet.
Angela Merkel, the German chancellor, is skipping the Olympics altogether.
The White House has said Bush would not boycott the Olympics because of the crackdown, arguing that the games are supposed to be about the athletes, not politics.
But Pelosi said "the president might want to rethink this later, depending on what other heads of state do".