U.S. Rep. Loretta Sanchez (D-Garden Grove) formally announced her candidacy to replace outgoing U.S. Senator Barbara Boxer (D) on Thursday.

The Orange County Democrat joins an ever-growing field of candidates, which includes: California Attorney General Kamala Harris (D), Oceanside Assemblymember Rocky Chavez (R), former GOP Chairman Tom Del Beccaro, and attorney Stewart Albertson (R).

The race will be the first time an open U.S. Senate seat will be decided under the state's new nonpartisan, top-two primary system, which was adopted in 2010. It will also be the first time in 24 years that one of the state's 2 Senate seats will be vacant.

In a state where 44 percent of registered voters are Democrats, a November 2016 election featuring two Democratic candidates is possible, but hasn't been seen before in a statewide race. In 2012, incumbent U.S. Senator Dianne Feinstein (D) didn't face much opposition from within her own party and easily defeated Republican Elizabeth Emken, 62.5 percent to 37.5 percent.

Sanchez made national headlines in 1996 when she upset incumbent Republican U.S. Rep. Bob Dornan in a very close election. The race came down to less than 1,000 votes and she has represented the Orange County area ever since.

Despite her notoriety, a statewide race presents unique challenges for Sanchez. The current favorite, Kamala Harris, has a clear fundraising advantage and has already held and been re-elected to a statewide office. However, the new dynamics in California can mean the race isn't as cut and dry as some pundits predict.