The incumbent Barack Obama has won re-election in the US presidential race, TV networks projected on Tuesday.

According to projections by major TV networks including Fox News, CNN and MSNBC, Obama has passed the threshold of 270 electoral votes needed to win the presidency, handily beating his Republican challenger Mitt Romney.

In a tweet to supporters, Obama wrote "This happened because of you. Thank you," as crowds cheered in Chicago headquarters, waiting for him to come out and address the nation.

Obama was projected to have won several key swing states, including Ohio, Pennsylvania, Wisconsin, Iowa, Nevada and New Hampshire, while Romney only won North Carolina. Florida and Virginia remained too close to call.

The relentless battle for the swing states gave Obama at least 284 electoral votes, while Romney just passed 200 before midnight.

Obama easily grabbed a host of "deep blue" states including California, Illinois and New Jersey, while Romney prevailed in "deep red" states of Texas, Kentucky and Georgia.

The elections also included contests for control of the Senate and House of Representatives. Projections indicated Democrats are to retain control of Senate, and Republicans will keep control of House.

Economic concerns loom large in the elections. Exit polls showed three out of five voters said the economy was the most important issue facing the country, with unemployment and rising prices most hurtful.

Related articles:

Obama wins re-election as US president