Party lines -- or lack thereof -- could mean no Republican challenger in the race for incumbent Dianne Feinstein's U.S. Senate seat in 2018.

We know state Senate Leader Kevin de León announced his bid for Feinstein's seat last month.

But, our state has a "top-two" system, meaning the two highest vote-getters in the June 2018 primary, regardless of political party, square off in the November 2018 general election.

Feinstein and de León are both Democrats.

"There's a very good chance that we'll have both of them," said University of California, Davis political sociologist Mindy Romero. "It's still really early."

Having two Democrats on the ballot isn't new, Kamala Harris defeated fellow Democrat Loretta Sanchez for U.S. Senate in last November's election, but that was an open U.S. Senate seat after incumbent Barbara Boxer decided not to seek re-election.

The top-two system only applies to statewide races and excludes the presidential primary.