Trump narrowly won the northern New Jersey's 11th District - situated just north of his golf club in Bedminster - in 2016 and tweeted a glowing endorsement that could fire up Republicans but perhaps alienate independents.
"He is outstanding in every way," Trump said. "Strong on Borders, loves our Military and our Vets. Big Crime fighter. Jay has my Full and Total Endorsement!
Webber responded in a statement that he's pleased to have Trump's backing and looks forward to working with him in Washington.
A message left with Sherrill's campaign was not immediately answered.
The race is among the most closely watched this year as Republicans defend their House majority and Democrats hope to flip 24 seats to take control.
Sherrill, a former Navy pilot and federal prosecutor, has Democrats optimistic they could capture the seat, long held by Republicans and open this year because Republican Rodney Frelinghuysen is retiring after 12 terms.
Polls have shown the race is close, though experts say they see an enthusiasm gap in favor of Democrats, who have registered more voters and raised more cash than the GOP in this race so far this year. Republicans have more registered voters than Democrats in the district.
David Wasserman, a House political analyst at the nonpartisan Cook Political Report, said Republicans have no choice but to use the president to try to motivate supporters. He noted that Webber has less cash than Sherrill and Republicans don't seem enthusiastic about voting in November, while Democrats "are on fire" with suburban women.
"Republicans are in deep trouble in this historically Republican seat," he said. But, he added, Trump's support could hurt as well as help.
"Trump's involvement is just as likely to alienate independents as rally Republicans," Wasserman said.
Sherrill has about $2.9 million cash on hand as of June 30, according to the Federal Election Commission, while Webber reported about $172,000.
Democrats also registered more voters so far this year, according to the state Division of Elections. They added about 7,000 voters through Sept. 4, according to the division. That's about a 4.5 percent increase, while Republicans added about 4,000 voters or roughly 2.4 percent more. Unaffiliated voters actually lost ground.
The northern New Jersey district includes parts of Essex, Morris, Passaic and Sussex counties.
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