Dianne Feinstein Kevin de Leon split
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.”

Obama and Feinstein were colleagues in the Senate and worked together extensively during the former President’s eight years in office.

Though Friday’s endorsement is not Obama’s first of the 2018 cycle – he backed Toni Preckwinkle for the Cook County Board earlier this year – it is his first in what are sure to be Democrats’ tough fight to take the Senate in 2018. Democrats are defending 26 Senate seats in 2018, with ten of those in states Trump won in 2016. Republicans are only defending nine seats.

De Leon announced last year that he would run against Feinstein, the 84-year-old Democrat who has served in the Senate since 1992.

So far his race, though, has been defined by his fundraising woes. De Leon has failed to tap into the base of Democratic donors who have long backed Feinstein, leaving the Democrat with around $600,000 in the bank.

Feinstein, on the other hand, has $10 million in her campaign war chest, money that will be needed in California, a state with multiple expensive media markets.

Billionaire Democratic donor Tom Steyer endorsed De Leon earlier this year, however, providing the candidate with a possible top donor to fund an outside organization to attack Feinstein.

“I have known Sen. De León for years and have fought alongside him on immigrant rights, expanding health care, and climate change,” he said. “Our work together on behalf of all Californians has assured me that he would be the champion of California’s priorities and values. Kevin de León has proven himself to be the best of the next generation, and I am proud to support him for U.S. Senate.”

Feinstein was also endorsed by the Los Angeles Times editorial board on Friday.

California has a top-two primary system, meaning it is possible Feinstein and De Leon could both advance in the primary and face off in the general election.