LOS ANGELES — The California Republican Party — a once dominant power in the nation’s largest state, the party of Earl Warren, Richard Nixon and Ronald Reagan — is teetering on the brink of irrelevance.

The Democratic sweep of Orange County congressional seats drew national attention on Election Day. But the Republican losses there were a symptom of the broader collapse of a storied political organization.

Republicans now hold just seven seats in the state’s 53-member congressional delegation after what shaped up to be a devastating midterm for the party. There was more bad news on Thursday when Representative David Valadao, a Central Valley Republican, conceded to T.J. Cox in what had been the one still-undecided district. Democrats are now likely to take 40 seats from Republicans as they decisively capture control of Congress.

Democrats captured three-quarters of the seats in the State Assembly, the biggest margin in over 100 years. The governor, lieutenant governor, both United States senators, the attorney general and the secretary of state are all Democrats. As of September, there were fewer registered Republican voters in California than Democrats or independents.