US Vice President Kamala Harris has officially won the Democratic Party's nomination to run in the upcoming US presidential election, after the party's national committee published the final results of a five-day online voting process that ended on Monday evening.

Harris is expected to announce Tuesday that she has formally accepted the party's nomination and chosen her running mate in the race for the White House, her first major decision as a Democratic presidential candidate.

Harris has narrowed her vice presidential running mate to Pennsylvania Gov. Josh Shapiro and Minnesota Gov. Tim Walz, sources told Reuters on Monday, after considering a group of candidates with a record of winning white, independent or rural voters.

A campaign official said no potential candidates had been contacted, but Harris was expected to appear with her running mate at an event in Philadelphia on Tuesday evening.

The chairman of the Democratic National Committee announced last week that Harris had secured the necessary number of delegates to become the party's nominee.

Kamala Harris has become the likely Democratic Party nominee for the presidential election, after President Joe Biden announced about two weeks ago that he was withdrawing from the race due to pressure he was subjected to from within his party, following his stumbling in a debate with Republican candidate Donald Trump.

In recent days, some opinion polls have shown Harris with a slight lead over Trump, but most polls indicate that they are tied at the national level, as well as at the level of the states described as swing states.