RALEIGH — A union representing North Carolina state employees has become involved quickly in the Democratic primary for lieutenant governor and plans to spend lots of money backing its favored candidate.

The State Employees Association of North Carolina said a radio advertisement supporting Linda Coleman was running starting Wednesday in Fayetteville, where competitor Eric Mansfield lives. Mansfield defeated an association-backed candidate in 2010 for the N.C. Senate.

Association executive director Dana Cope said the association’s Board of Governors decided last weekend to invest significantly to support Coleman. Cope suggested the amount from national and state sources could reach $1.8 million — a huge amount for a lieutenant governor’s race.

Mansfield spokesman Thomas Mills declined comment on the amount but questioned the association spending money even before candidate filing ends next week.