Democrats have retained their Senate majority, according to AP projections.

Rep. Mazie Hirono's (D) apparent win in Hawaii means Democrats have secured 50 Senate seats with six states still outstanding — Wisconsin, North Dakota, Montana, Nevada, Arizona and New Mexico.

Because most are projecting that President Obama has won reelection, that means Vice President Biden would cast any tie-breaking voters in the Senate.

Democrats have a chance to add to their majority in all six states that have yet to be called. The first four are considered tossups, while Arizona leans Republican and New Mexico leans Democratic.

Democrats have won every competitive race that has been called tonight except Nebraska. They won in Hawaii, Michigan, Pennsylvania, Missouri, Indiana, Florida, Virginia, Ohio, Connecticut, Maine and Massachusetts.