The campaign for GOP candidate for Congress in New Jersey's 11th Congressional district, Assemblyman Jay Webber, R-Morris, said Wednesday that a threat against him and his seven children was delivered to his office in Whippany yesterday.

A note sent to Webber's office warned Webber "You better hope you don't win! Or else." and then asked, "How many kids do you have...7? Unlucky 7."

Webber has frequently incorporated his wife Johanna and their seven children into his campaign, featuring them in campaign ads.

The threat, which arrived with a cut up Webber for Congress sign, has been referred to the state police for investigation, according to Ronica Cleary, a spokesperson for the campaign.

The race for New Jersey's 11th congressional district, which includes Morris and Essex Counties, is hotly contested and closely watched. U.S. Rep. Rodney Frelinghuysen, R-11, held the seat for decades never winning by less than 19 points, but Hillary Clinton lost the district by a single point in 2016. National Democrats view it as a key district in their goal to gain control of the House of Representatives.

An Oct. 9th poll by Monmouth University has Webber's rival, Democrat Mikie Sherrill, ahead in the race by four points, 48 percent to 44 percent, but within the margin of error.

On Wednesday, Webber took to twitter to decry the ugly turn the race has taken.

Criticizing a candidate on the issues is part of politics. Threatening my children is not. We are thankful for the support of law enforcement as we work to find whoever is behind this gutless act. We will not be intimidated as we work to make #NJ11 better for everyone. pic.twitter.com/nmVRuV9ATH — Jay Webber (@JayWebberNJ) October 24, 2018

The threat arrived by US Mai at Webber's Whippany law office a day after Webber's campaign tweeted that his opposition on the political left was "unhinged" and posted a video showing torn up and defaced Webber campaign quotes intercut with renowned Democrats like former Speaker of the House Nancy Pelosi and Hillary Clinton advocating direct action by voters.

WATCH: The angry left is fully unhinged. Maybe b/c the left’s leaders in Washington encourage violence! Don’t retaliate! Respond by putting up more #WebberForCongress signs & volunteering. Let's win this the right way & work together to make #NJ11 better for everyone. pic.twitter.com/zgw6f4IMxm — Jay Webber (@JayWebberNJ) October 18, 2018

Webber campaign spokeswoman Ronica Cleary released a statement to media on Wednesday:

"We are thankful for the immediate responsiveness and support of law enforcement in light of this threat. Vehement criticism of a candidate on issues of public concern is part of politics, but when anonymous individuals menace a candidate and his family, things have gotten out of hand."

In a statement released Wednesday afternoon, Webber's Democratic opponent, Mikie Sherrill, herself a mother of four, blasted the threat as "absolutely appalling."

"No one who decides to run for public office should feel that, by doing so, they are putting their family in danger," said Sherrill. "I condemn this in the strongest terms and hope law enforcement determines who was behind this and holds them accountable."

Anyone with information about the incident should contact state police's regional operations intelligence center's Watch and Warning Unit at (609) 963-6900, option 1.

Claude Brodesser-Akner may be reached at cbrodesser@njadvancemedia.com. Follow him on Twitter @ClaudeBrodesser. Find NJ.com Politics on Facebook.