U.S. Rep. Michael Castle, the choice of the GOP establishment in Tuesday's primary, is trying to fend off a possible upset by tea party candidate Christine O'Donnell.
State party spokesman Tom Doheny said Monday that chairman Tom Ross, who supports Castle and has denounced O'Donnell as a liar and a fraud, last week received a vulgarity-laced e-mail.
Doheny declined to provide a copy of the e-mail or divulge the address it came from, but said the writer suggested that Ross and other so-called Republicans in Name Only, or RINOs, deserve "a bullet in the head."
Doheny said Ross took the threat seriously enough to move his family out of their home, and that the Department of Justice is investigating.
"Obviously there's a lot of growing anger and tension out there," Doheny said.
DOJ spokeswoman Tracy Schmaler declined to comment.
From the start of her campaign, O'Donnell has criticized Castle, a leader of GOP moderates, as the most liberal Republican in Congress and "king of the RINOs."
On Sunday, Amy Kremer, the chairwoman of the Tea Party Express, demanded that Ross resign as party chairman within 24 hours or "face immediate termination."
But Kremer, who supports O'Donnell, said she meant he should resign or be fired and did not know anything about the death threat until being told about it by The Associated Press.
"I would absolutely renounce that; I mean that's ridiculous. That's disgraceful and it shouldn't be tolerated," Kremer said.
O'Donnell's campaign said it was not responsible for the threat and the candidate, in a statement, condemned threats of violence.
"We hope Mr. Ross and his family are safe, as no one should have to go through personal attacks like this," the statement read.
Castle did not return several requests for comment left with his campaign.
He and O'Donnell are competing to run for the seat vacated by Democrat Joe Biden when he became vice president. The winner will face Democrat Chris Coons in November. Democrat Ted Kaufman, who was appointed to replace Biden temporarily, is not running.
The GOP has been harshly critical of O'Donnell as it tries to avoid a repeat of what happened in Alaska when tea party candidate Joe Miller upset Alaska Sen. Lisa Murkowski in the Republican primary there. The party sees Castle as its best chance to take the Delaware seat from Democrats.