Sen. Dianne Feinstein (D-Calif.) holds an 18-point lead over her progressive challenger, California state Sen. Kevin de León (D), ahead of November's midterms, according to a new poll.

Feinstein has the support of 41 percent of likely voters to de León's 23 percent, according to an Emerson College poll released Monday. The poll, which has a margin of error of 4.1 percentage points, found that 37 percent of likely voters remain undecided.

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Feinstein and de León are both Democrats but are facing off in the general election after they both advanced in California's jungle primary earlier this year.

Most of the undecided voters are made up of Republicans, with 61 percent of voters in that party saying they haven't yet made up their mind, the poll found.

The California Democratic Party in July endorsed de León in what was seen as a rebuke of Feinstein.

De León earlier this month called on Democrats to "stop biding our time and biting our tongue." He also said the Democrats in Washington frequently "lack the courage of their convictions."

The Emerson College poll's results were based on interviews and surveys with 671 likely voters in California from Oct. 17-19.