(CNN) Former President Barack Obama endorsed Sen. Dianne Feinstein on Friday, his first Senate endorsement of the 2018 cycle.

By backing Feinstein, Obama is stepping into what could be a contentious battle between the incumbent Democrat and California Democratic state Senate president Kevin de León, a liberal upstart who is looking to unseat the longtime senator.

"She's always been an indispensable leader for California, and we became dear friends and partners in the fight to guarantee affordable healthcare and economic opportunity for everybody; to protect our planet from climate change, and our kids from gun violence," Obama said in a statement. "I ask Californians to join me in supporting Dianne Feinstein's reelection and returning one of America's most effective champions for progress to the Senate."

Feinstein said it "means a great deal to me to receive President Obama's endorsement, and I'm thankful and honored for his trust."

"Barack Obama was a singular President with a remarkable legacy that will truly stand the test of time" she said. "President Obama had the grace, wisdom and even-handedness that we quickly came to expect from a president -- and that we're now so sorely disappointed by its absence. I'll do my level best every day to build on President Obama's accomplishments and carry his torch forward, no matter the obstacles that stand in our way."

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