California voters will elect a U.S. senator on Nov. 8 in a historic Democrat-on-Democrat race featuring state Atty. Gen. Kamala Harris and Orange County Rep. Loretta Sanchez. It's California's first U.S. Senate contest with no Republican on the general election ballot since the state began direct election of senators in 1914.

The two Democrats are facing off in the general election due to Proposition 14, which California voters passed in 2010. Also known as the Top-Two Primaries Act, the law requires candidates from any party to run in a single primary. The top two vote-winners advance to a runoff in the general election, regardless of party affiliation. Three other states — Louisiana, Nebraska and Washington — have similar systems.

This isn't the first time nationwide that two candidates of the same party have met in a general election contest because of a voter-approved top-two primary system. For example, during the 2010 Washington House of Representatives race, Republican candidates faced off against each other in the general election. Washington passed Initiative 872, its version of the Top-Two Primaries Act, in 2004.

USC public policy professor and political expert Dr. Dora Kingsley Vertenten believes this election is historic because it has led to an obliteration of Republican candidates, as well as a breakdown within the Democratic Party.

"If it was up to me I would continue with the older model, which is one Republican and one Democrat or whatever the two political parties are that we choose to support. I do not support an election that has candidates from the same party in a general election," Kingsley Vertenten said.

President Obama and Vice President Joe Biden announced their endorsement of Harris for U.S. Senate in July in a statement released by the Harris campaign and Democratic National Committee. Additionally, Harris has gained the support of California Gov. Jerry Brown, who announced his endorsement at the headquarters of the California Democratic Party in May.

Harris beat Sanchez in the primary by more than 800,000 votes, according to Ballotpedia.

Kingsley Vertenten thinks high-profile endorsements are only part of the equation that help candidates win an election. With two candidates from the same party, the deciding factor will be how the general election voters believe a candidate is going to conduct themselves in office.

"I think that the Congresswoman [Sanchez] from Orange County is not as well known and perhaps has not promoted herself or the kind of legal reform issues that are important to the voters in California," Kingsley Vertenten said. "I think it's not unexpected that she is doing poorly, and in the public opinion polls, she is likely to not prevail in two weeks."

A September Field poll found that 42 percent of likely voters in California backed Harris, compared to 20 percent for Sanchez. The others are undecided or will not vote for either candidate.

Correction: A previous version of this story stated that Louisiana, Nebraska and Washington implemented the "Top-Two Primaries Act." While those states use a top-two primary system, that act only refers to the California ballot proposition.

Reach Staff Reporter Julia Adams here.