(CNN) The Republican Party has been left out of a race for one of the most powerful political seats up for grabs in the most populous state in America.

It's Democrat versus Democrat for California's U.S. Senate seat this November, a general election match-up that has never happened before.

But it will likely happen again political analysts say all because of a rule authored by a Republican, signed by a GOP governor and passed by California voters six years ago.

Following Sen. Barbara Boxer announcement last year that she'd end her more than 20 years in the Senate at the conclusion of her term, at least 34 candidates -- including 12 Republicans in the typically blue state -- threw their names into the race.

As the June 7 primary drew nearer, three candidates became the clear front-runners: Two Democrats -- California Attorney General Kamala Harris and U.S. Representative Loretta Sanchez -- and one Republican: former Republican State Party Chairman Duf Sundheim.

When the votes were counted the two front-runners were obvious: Harris with 40.4% the vote and Sanchez trailing but clearly second with 18.6% of the vote. Sundheim came in third with 8% of the vote. Instead of the top Republican and top Democrat going at it in the general election, it will be two Democrats.

How did a two-party race end up being a one-party affair?

The Republicans have themselves in part to thank for that.

What would have once been a political impossibility turned into a reality because of a proposition put on the ballot in 2010. Proposition 14 was a constitutional amendment that made it so the top two vote-getters no matter which party they are from would qualify for the general election.

It was authored by Republican State Sen. Abdel Maldonado and backed by then-Gov. Arnold Schwarzenegger, a Republican. Voters got a chance to weigh in during fall 2010 and passed the measure.

"California, like much of the country, was becoming more and more polarized and more and more partisan, so several years ago, a number of centrists in both parties thought that by implementing a 'top two' primary it would encourage more centrist candidates to run and succeed," said Dan Schnur, director of the University of Southern California Unruh Institute of Politics.

What that has meant for the U.S. Senate race in California is a historic scenario. For the first time in California history, a Democrat is up against another Democrat for the Senate seat in the general election.

So far, Harris has been leading the polls. If she wins, Harris, who is of black and Asian decent, would become the second black woman in the U.S. Senate. She has the endorsement of the state party and Democratic Gov. Jerry Brown.

But Sanchez has beaten the odds before. Nearly 20 years ago, she was up against an entrenched Republican and was not a favorite to win the House seat. But she ended up the victor and has been in Congress ever since.

Now she faces another battle as the underdog. The key to her success could lie in her ability to reach across the aisle and appeal to Republicans.

That is exactly the kind of thing that proposition 14 was meant to encourage.

"The first thing she has to do is educate Republican voters that she is the moderate of the two candidates," Schnur said. "Her other priority is to motivate her strongest base of supporters to come out to the poll and numbers show that has been the Latino vote."

No matter who wins, history will be made. If Sanchez pulls out a win in November, U.S. history will be made twice. Sanchez is the daughter of Mexican immigrant parents. She would become the first Latina voted into the U.S. Senate.