Donald TrumpDonald John TrumpReporters defend CNN's Acosta after White House says he 'disrespected' Trump with question Security costs of Trump visit to Scotland sparks outrage among Scottish citizens Ex-CIA officer: Prosecution of Russians indicted for DNC hack 'ain't ever going to happen' MORE endorsed Rep. Darrell Issa (R-Calif.) for reelection on Tuesday, as the former House Oversight Committee chairman faces the toughest reelection fight of his career.

.@DarrellIssa is a very good man. Help him win his congressional seat in California,” Trump tweeted.

It’s unclear whether the endorsement, which comes a week from Election Day, will be helpful to the California Republican. Issa, a 15-year incumbent, is in a more competitive race than expected largely because of his support for Trump, the Republican presidential nominee.

Democratic challenger Doug Applegate has been tying Issa to the GOP nominee in campaign ads. Polling conducted by Democrats has shown Trump trailing behind Democratic nominee Hillary ClintonHillary Diane Rodham ClintonDHS secretary: No sign Russia targeting midterm elections at 2016 level Twitter suspends Guccifer 2.0 and DCLeaks accounts after indictments Elon Musk donated nearly K to Republican PAC, filings show MORE, despite Republicans’ 8-point voter registration edge in the suburban San Diego district. 

The affluent district has become increasingly diverse in recent years, adding demographics that polling has shown have unfavorable views of Trump.

Issa acknowledged in a recent interview with a local Fox affiliate that “a majority of my district will probably go for Hillary Clinton.”

Issa is best known for his five years leading House Oversight Committee investigations into the Obama administration from 2011 to 2015. 

Lately, Issa has been claiming bipartisan credentials with a mailer touting his support of a bill to help sexual assault survivors that included a photo of President Obama. 

Obama called the move “shameless” at a recent fundraiser to benefit Applegate.