Steyer and Harris would be among the most formidable contenders for Boxer`s seat, along with former Los Angeles Mayor Antonio Villaraigosa, who has said he is "seriously considering" a run, according to CNN.

Harris, the former district attorney of San Francisco who was just re-elected to her second term as attorney general, was the first to formally declare her candidacy since Boxer announced last week that she would not run for re-election in 2016.

Harris` position was strengthened on Monday when Lt Governor Gavin Newsom bowed out of contention. Newsom, 47, is reported to be interested in running for governor when Jerry Brown faces term limits in 2018. If Harris wins, she would be the first ever Indian-American in the 100-member U.S. Senate.

Three Indian-Americans have been elected to the U.S. House of Representatives: Dalip Singh Saund, Bobby Jindal and Ami Bera, who was re-elected for his second term last November. There are also currently two U.S. governors of Indian origin - Bobby Jindal and Nikki Haley - both Republicans are said to be mulling presidential runs in 2016. Obama last year caused a bit of a stir when he called Harris "by far the best-looking attorney general."

He later called long time friend Harris to apologise for saying: "She`s brilliant and she`s dedicated, she`s tough. She also happens to be, by far, the best-looking attorney general ... It`s true! C`mon." Harris graciously accepted the President`s apology.

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