Sen. Kamala Harris, D-Calif., left, and California gubernatorial Democratic candidate Gavin Newsom applaud as they listen to Katie Porter, a congressional candidate for California's 45th District, during a campaign rally Saturday, Nov. 3, 2018, in Irvine, Calif. The fight for control of the U.S. House is attracting big names to historically Republican Orange County, California. (AP Photo/Jae C. Hong)

Two top California Democrats – once viewed as rivals – hit the campaign trail together in the Golden State over the weekend to stump for candidates challenging Republican incumbents in the House.

Lt. Gov. Gavin Newsom, who enjoys a sizeable lead in this year’s gubernatorial race, and Sen. Kamala Harris are looking to use their growing star power in the Democratic Party to rally voters to the polls in traditional Republican strongholds.

The pair campaigned in four key regions of the state that voted for Democratic presidential nominee Hillary Clinton in 2016 despite being seen as Republican districts. In places like Orange County and the San Fernando Valley – both places where the Latino vote is seen as crucial for both parties – Newsom said President Trump was using the migrant caravans currently making their way through Mexico as a tool to fire up his Republican base.

“This is a state that has brought in 112,000 refugees in the last 10 years... We celebrate that diversity,’’ Newsom said during a rally in Santa Clarita for Democrat Katie Hill. Hill is challenging incumbent Republican Steve Knight in California’s 25th congressional district.

Newsom added: “That’s what makes California great, that’s what makes America great... This is our moment. This is your moment.”

Newsom is able to stump for congressional Democratic candidates despite himself running for the governor’s house in America's most populous state because of the sizeable lead he holds over Republican businessman John Cox. He's up by about 20 points, according to recent polls.

Harris, a first-term senator who took office in 2017, also has been busy on the campaign trail this midterm season. She has made recent stops in South Carolina and Iowa and has raised $2.4 million for the Democrats this election.

There is widespread speculation that Harris could be using her time on the campaign trail this fall to test the waters for a presidential run against Trump in 2020.

“Kamala’s got something going on, objectively... in the fact that she is where she is, in that top tier,’’ Newsom told Politico.

The final days of the election cycle have brought a number of high-profile Democrats out onto the campaign trail.

Former Vice President Joe Biden – widely viewed as the Democratic frontrunner for the 2020 presidential nomination – has criss-crossed the nation to stump for candidates in his party, while former President Barack Obama recently has taken on a more public role this fall after refraining from offering a full-blown counterpoint to Trump's policies,

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Obama was traveling over the weekend to Gary, Indiana, on behalf of Sen. Joe Donnelly, D-Ind., who faces a stiff challenge from Republican businessman Mike Braun. The former president's schedule also includes a stop in Chicago to campaign for businessman J.B. Pritzker, the Democrats' nominee for Illinois governor.

Obama has accused Trump of lying and "fear-mongering" and warned Democrats not to be distracted.

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Trump has punched back, accusing Obama of leaving behind a trail of broken promises on trade, the economic recovery and a promise during his presidency that patients could keep their doctors under his health-care law.

Trump was holding Sunday afternoon rallies in Macon, Georgia, and Chattanooga, Tennessee, in support of Republican Brian Kemp, who is running for Georgia governor, and Rep. Marsha Blackburn, who is seeking an open Senate seat in Tennessee.

The Associated Press contributed to this report.