The 49th Congressional District race has attracted significant attention, one of a handful of Southern California contests in which national Democrats are heavily investing as part of their goal to flip the U.S. House of Representatives on a wave of anti-Trump sentiment.

But the sense of optimism among Democrats has faded in recent weeks over fears that the district is slipping away — and that it’s their own fault.

Rep. Darrell Issa’s announcement in early January that he would not seek re-election caused a free-for-all of candidates and completely changed the dynamics of the race. Behind the scenes, leading contenders have tried convincing one another to drop out, arguing that having too many Democrats on the primary ballot could lead to two Republicans in the November general election.

At the same time, the Service Employees International Union vowed to hold a forum with the Democratic contenders in coming weeks and publicly pressure those who are trailing in the polls to step down.

Three interesting things happened next.

On Feb. 9, county officials confirm, Democrat Doug Applegate changed his primary residency from San Clemente to Oceanside — creating the possibility of running for the San Diego County Board of Supervisors’ 5th District.

That was two days after SEIU opened an independent expenditure committee in San Diego County to support him, should he decide to run. The only problem was that state law required he establish primary residency in the county district at least 30 days before the nomination period closed. The filing deadline is March 9.

Applegate’s campaign declined to comment for this story.

The well-heeled union is deeply involved in county politics because many of its members work for the county government. In 2014, the union supported Republican Oceanside Mayor Jim Wood over Bill Horn in the 5th District through an independent expenditure committee.

This week, the only Democrat running for the county’s 5th District, Oceanside City Councilwoman Esther Sanchez, finally responded to questions of whether her campaign bid was sincere. She raised no money in the final quarter of 2017, and Democrats had begun talking about which Republican they’d like to see replace Horn, who has held the seat since 1994 but is termed out.

On Facebook, Sanchez confirmed that she had recently dropped out of the county race. “I left to make room for another candidate, as it is definitely a winnable seat for someone other than the 2 now running,” she wrote Tuesday.

It’s not clear to whom Sanchez was referring. She did not return a request Thursday for more information.

As of Thursday afternoon, there were no Democrats listed for the race on the local party’s website.

A poll released Wednesday from the Democratic group Flip the 49th — to which SEIU is a donor — again raised concern that none of the congressional Democratic candidates would advance through June. In California primaries, the top two candidates advance to the general, regardless of party. Republicans Diane Harkey, a member of the Board of Equalization, and Assemblyman Rocky Chávez are polling atop the field after respondents heard negative messages about the others.

Other Republicans in the congressional race include San Diego County Board Supervisor Kristin Gaspar, San Juan Capistrano mayor pro tem Brian Maryott and attorney Joshua Schoonover.

The poll’s methodology has raised criticism that it was intended to cull the Democratic field, which includes at least five others: Sara Jacobs, Martin Kent, Paul Kerr​, Mike Levin and Christina Prejean.

“The scope of positive information provided about each candidate was calibrated to consider resources, so Applegate received a limited bio and Prejean received none,” a memo on the poll reads.

Applegate doesn’t have the campaign funds available to fellow Democrats Levin and Jacobs.

A retired Marine Corps colonel, Applegate ran for the 49th District seat against Rep. Darrell Issa in 2016 but lost by 0.6 percent. He has maintained a high level of support among the party’s activist base, even as he competes for the party leadership’s support.

Delegates for the California Democratic Party met on Jan. 28 to make a pre-endorsement and gave the nod to Levin, an environmental attorney, although he failed to get enough votes to secure the endorsement — a threshold of 60 percent. Applegate came in second.

Late last week, though, his campaign hyped the results of a KGTV and Union-Tribune poll suggesting he was the Democratic front-runner among likely voters and competing neck-and-neck with Chavez.

Five days later, Cosmopolitan magazine published a profile of Jacobs, a former volunteer adviser to Hillary Clinton’s 2016 presidential campaign. It gave a sense of the hostilities between Democratic contenders. She called Applegate a “crusty old Marine.” Later, she issued an apology.

Correction: An earlier version of this story said Mike Levin’s campaign was supported by his personal wealth. He has not loaned his campaign any money.