WASHINGTON (AP) — Two health care unions are joining forces in a move that could threaten the dominance of a powerful rival and lead to a new round of labor tensions.

The 185,000-member California Nurses Association is merging with the 10,000-member National Union of Healthcare Workers. They're forming a new union of health sector workers.

The merger renews a bitter rivalry between the nurses and the 2 million-member Service Employees International Union. The SEIU is the nation's dominant health care union and a major force in Democratic politics.

The merger also points to a trend that could see unions increasingly competing against each other for a dwindling pool of members.

A priority for the new alliance is to lure 43,000 unionized workers at Kaiser Permanente away from the SEIU.