Democrats’ all-out effort to win a handful of GOP-held seats in California to take control of the House is still a messy scrap among a crowded field of candidates -- with the latest incident a tough-talking, unsubstantiated voicemail one candidate allegedly left on a rival candidate’s phone recorder.

The call was reportedly made in recent days by Gil Cisneros to Andy Thorburn -- the top Democrats in a field of 17 candidates vying for the Orange County seat of retiring GOP Rep. Ed Royce.

“Hi Andy, it’s Gil Cisneros,” a man’s voice is heard saying. “I’m gonna go negative on you.”

The Cisneros campaign said Monday that a report by independent forensic analyst “clearly” states that the unidentifiable voice was not the candidate’s.

“The voice on that recording is not Gil Cisneros, period," said campaign spokesman Orrin Evan about the tape, first reported by The Intercept blog.

Washington Democrats have for months worked behind the scenes to winnow their field of candidates -- attracted to the races in Southern California and other parts of the state by changing demographic and Democrat presidential candidate Hillary Clinton winning in seven districts in 2016 that had voted Republican since the Depression era.

Some candidates have dropped out, realizing that their campaigns would only hurt their party. California has a top-two primary system, meaning the No. 1 and No. 2 vote-getters in the June 5 primaries advance to the general election regardless of party. And numerous Democratic candidates would splinter votes and perhaps put two Republicans in the November elections.

In Royce’s district, Cisneros, a Navy veteran and $266 million lottery winner who’s loaned his campaign $2 million, appears to be the establishment pick.

Last week, the Democratic Congressional Campaign Committee, the party's national House campaign arm, included him in its most recent batch of top-notch “Red to Blue” candidates, signaling to donors that he is the most viable candidate among the six Democrats seeking the seat against seven Republicans.

“Beyond their demonstrated abilities to build a winning campaign infrastructure, these candidates have strong records of service and deep ties to the communities they aim to represent,” DCCC Chairman Rep. Ben Ray Lujan, of New Mexico, said of the Red to Blue group.

Thorburn, a former insurance executive, appears a favorite of liberals, with endorsements from nurses who want universal health care and Our Revolution, a Bernie Sanders-inspired political committee. He's loaned his campaign more than $2 million. And pediatrician Mai Khanh Tran, another Democrat in the field, has pumped $480,000 into her campaign.

Republicans have established names on that ballot, including former state Senate leader Bob Huff, Orange County Supervisor Shawn Nelson and former State Assembly member Young Kim, a former Royce aide who has his endorsement.

Meanwhile, incumbent Republican Rep. Dana Rohrabacher faces four party challengers and six Democrats in the 48th District. And four Democrats and eight Republicans are running to replace Republican Rep. Darrell Issa, who is retiring in the 49th District.

"This is a free for all. This is the Wild West," Jessica Hayes, chairperson of the San Diego County Democratic Party, said of the 49th District race.

Democrats emboldened by an unpopular president and a diversifying population, including more Hispanic voters in Orange County, hope to capture all four of the Republican-held U.S. House seats in the county, and three more in other parts of the state.

They need to gain a total of roughly 24 seats to take control of the House. They've been heartened by recent Democratic victories in Pennsylvania and Alabama, and House Speaker Paul Ryan's decision to retire at the end of the term has fanned the headwinds faced by the GOP.

Republican leaders, hoping to retain control of the House, have opened a 10,000-square-foot war room in Orange County, near John Wayne Airport, and filled it with computers, phones and, on one day last month, dozens of volunteers making calls to potential primary voters.

To Ohio Rep. Steve Stivers, chairman of the National Republican Congressional Committee, the GOP majority runs through Orange County.

It's in suburbs that "we are going to either hold the majority in '18 or lose the majority," he said.

The risks are plain for Republicans in the state that is home to House Minority Leader Nancy Pelosi: Democrats dominate California politics, and midterm elections generally favor the party not in control of the White House. President Trump lost the state by more than 4 million votes in 2016 and there's no sign he's gained support since then.

"I'm profoundly concerned about my kids growing up in Donald Trump's America," David Min, the endorsed Democratic candidate in the battleground 45th District held by Republican Rep. Mimi Walters, says in an online video.

The Associated Press contributed to this report.