SACRAMENTO -- The field of candidates competing for the coastal Senate district seat being vacated by Ted Lieu grew Wednesday with former Assemblywoman Betsy Butler and Manhattan Beach Mayor Amy Howorth, both Democrats, announcing their candidacies.

They join other Democratic candidates who previously announced plans to compete for the 26th Senate District seat: women’s rights activist Sandra Fluke, Santa Monica-Malibu school board member Ben Allen and Vito Imbasciani, a state surgeon for the California Army National Guard.

No Republican has announced a run for the seat.

Butler was planning to run for the Assembly, but switched to the Senate seat after Lieu announced plans to compete for the congressional seat being vacated by retiring Rep. Henry Waxman.

Butler has lived in the Senate district for 20 years. A resident of Marina del Rey, Butler said she served a large portion of the district when she was in the Assembly for two years ending in 2012. Her endorsers include California state senators Ricardo Lara, Mark Leno, Holly Mitchell, Marty Block, Ed Hernandez, Ben Hueso and Noreen Evans, she said.

“California is facing many challenges but we are resilient and committed to finding the answers on a wide range of issues including water, environmental justice, equality, opportunity for all and an investment in the future of our children,” Butler said in a statement.

Howorth touted her 17 years in the district, during which she raised her family and served as a PTA activist, a Democratic Party delegate, a school board member, a city councilwoman and mayor.

“This district needs a representative in the Senate who has deep roots and close connections to its many neighborhoods and communities, and who also reflects its local values,” Howorth said. “I am that candidate.”

Howorth was endorsed Wednesday by Assemblyman Al Muratsuchi (D-Torrance), who represents about 31% of the Senate District.

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patrick.mcgreevy@latimes.com