 -- Democratic state Sen. Kevin de León announced on Sunday his bid to challenge longtime Sen. Dianne Feinstein, who at 84 is the oldest member of the Senate, in the 2018 primary election. He is the first candidate to announce a challenge to Feinstein.

"I'm running for U.S Senate to expand the California Dream (and) unite this country along progressive values," California Senate President pro Tempore de León, 50, wrote in a tweet about his decision, first announced in an email to supporters.

In a video accompanying both his tweet and email, de León discussed how growing up as "the youngest child of a single immigrant mother," and seeing his mother work "her fingers to the bone" every day cleaning houses, led him to this point in his life.

"I think that it's incumbent on policymakers, on leaders, to provide real opportunity so everyone can succeed," he said in the video. "That's what the country really wants. They want a job. They want a sense of security, so they can provide for themselves and their family."

De León already won the immediate endorsement of Democracy for America, which has previously endorsed more liberal members of Congress, including Sen. Bernie Sanders, I-Vermont, and Sen. Elizabeth Warren, D-Massachusetts. Nearly a third of the organization's members live in California.

DFA's Executive Director Charles Chamberlain said in a statement, "When Trump became President, de León didn't ask people to sit back and wait, hoping maybe Trump would someday turn out OK. He immediately began working to pass policies that would help the people most immediately affected by Trump's bigoted, greedy policies. The simple fact is this: We won't defeat Trump and his Republican Party with corporate Democrats pushing Republican-lite policies and weak leadership."

"Earning the early support of Democracy for America's grassroots members isn't just an honor, it's a reflection of my commitment to running a people-powered campaign for the U.S. Senate," de León said about the endorsement, according to an email from DFA.

According to his website, De León was elected to the California Senate in 2010 after serving three years in the state's General Assembly. He was elected to lead the 40-member body in 2014, becoming the first Latino in 100 years to hold the position.

On Monday, Feinstein announced she would seek a fifth full term, writing on Twitter, "I am running for reelection to the Senate. Lots more to do: ending gun violence, combating climate change, access to healthcare. I'm all in!" Feinstein joined the Senate in November 1992 when she won a special election to replace then-Sen. Pete Wilson, a Republican who had been elected governor.

"Sen. Feinstein is getting ready to run a very big race, well financed and organized campaign, and she comes to the campaign with a strong record of success in California elections," Bill Carrick, longtime political consultant for Feinstein, told ABC News.

Carrick noted that Feinstein already has endorsements from Democrats Sen. Kamala Harris, Rep. Adam Schiff, Lt. Gov. Gavin Newsom, California Secretary of State Alex Padilla among other California officials. He also said that Feinstein has "strong support" among women, Latino, African American and Asian-American voters.

"We think she's in good shape," Carrick said. "Kevin de León has never run statewide for anything. It remains to be seen what kind of candidate he will be or campaign he would run."