Trump's endorsement boosts Jack Ciattarelli in New Jersey's Republican primary, but Bill Spadea's campaign sees a surge in grassroots support and fundraising.

But fealty to Mr. Trump is not without risk in November’s general election. Democrats control New Jersey’s State House and Democratic voters still vastly outnumber Republicans, despite the G.O.P.’s recent success in narrowing the gap to about 800,000 voters.