LOS ANGELES — The campaign to replace liberal icon Barbara Boxer in the U.S. Senate is providing a glimpse into a one-sided political future in California, in which voters could increasingly find only two Democrats to pick from for top offices in November elections.

Democrats have dominated California politics for years; the party holds every statewide office and controls both chambers of the Legislature.

But it’s the first time in the modern era that a Republican will not appear on the Senate ballot. The choice for voters is two Democratic women, state Attorney General Kamala Harris or U.S. Rep. Loretta Sanchez.

The Democrats-only runoff, created by the state’s unusual primary election rules where the top two vote-getters advance regardless of party affiliation, is likely to become a more frequent outcome in statewide races as Republican registration numbers continue to decline.

“This race could symbolize the end of two-party politics in California,” said Thad Kousser, chairman of the political science department at the University of California, San Diego.

The one-party dynamic has reordered the low-key contest, all but erasing public interest in what will be a certain Democratic victory while affecting everything from fundraising to strategy for the candidates.

It’s also likely to make it harder to recruit Republican candidates for future statewide elections by reaffirming the state’s prominent Democratic tilt.

“The psychological impact of this race isn’t good for the Republicans,” said Claremont McKenna College political scientist Jack Pitney.

“It’s a preview of what could happen” in 2018, when the governor’s seat and other statewide offices will be contested, Pitney added.

With only Democrats on the ballot, and Harris the favorite of President Barack Obama and the Democratic Party establishment, Sanchez has attempted to defy the laws of political physics and straddle the wide gulf between the two parties.

The Orange County congresswoman needs to cobble together an unusual coalition, with a base of Southern California Democrats, fellow Hispanics and independents. She’s also been seeking Republican votes by stressing her credentials on national security and small business, issues that often resonate with GOP voters.

Suddenly an outsider in her own party, Sanchez, 56, has criticized Obama for endorsing Harris and depicted her rival as entrenched in a failed “political establishment.”

Harris, 51, was the first candidate to enter the race and has never trailed in fundraising or polling.

The race has recently become an unwelcome sight for many Democrats, with the candidates and their campaigns exchanging sharp attacks. Harris’ campaign has criticized Sanchez for missing a string of House votes, while Sanchez has argued that the prosecutor Harris talks but rarely delivers.

“I don’t like to see (Democrats) doing that, but with the shrinking population of Republicans, that’s inevitable,” lamented Alexandra Gallardo-Rooker, a vice chairwoman of the California Democratic Party who is supporting Harris.

The candidates agree on many issues, including the need for immigration reform, battling climate change, cutting college costs, defending abortion rights and the $15 minimum wage.

Sanchez argues that her tenure on the Homeland Security and Armed Services committees gives her experience needed in an era of global threats, which her opponent cannot match. Seeking to close the gap with Harris, she has faulted the attorney general for rising crime rates, and argued that a vote for Harris is an endorsement of the status quo. She has also urged voters to reject Proposition 57, Gov. Jerry Brown’s proposal to allow some prisoners serving time for nonviolent crimes to be considered for early release.

Harris, as attorney general, typically does not take positions on ballot measures, her office said. She has promoted her record as a lifetime prosecutor, including taking on banks accused of improper mortgage foreclosures. Her campaign has repeatedly criticized Sanchez for missing House votes.

There were 34 candidates on the ballot in the Senate primary in June and Harris won easily with about 40 percent of the vote to Sanchez’s 19 percent. None of the dozen little-known Republicans on the ballot was able to get out of single digits.

The Democrat-against-Democrat contest has historic overtones: Sanchez, if elected, could become one of the first Latinas to hold a U.S. Senate seat and Harris could become the first Indian woman and the second black woman elected to the chamber. Harris’ father is Jamaican and her mother is from India.

Carol Moseley Braun is the only black woman elected to the Senate. Illinois voters chose her in 1992 and she served one term.