NEW YORK — Longtime labor leader George Gresham was toppled by his former lieutenants in a contentious race to lead the nation’s largest health care union.

Gresham, who became president of 1199SEIU United Healthcare Workers East in 2007, lost his reelection bid by a resounding margin to challenger Yvonne Armstrong, who leads the union’s long-term care division, according to data reviewed by POLITICO.

Gresham’s defeat ushers in a new chapter for the union, which represents 450,000 health care workers across five East Coast states and was once a powerhouse in New York politics.

It is very rare for union heads to lose internal elections in New York, a state in which labor holds outsize power in local politics. 1199SEIU helped elevate Bill to Blasio to the mayoralty when he was lagging in the polls 12 years ago, and it recently endorsed Andrew Cuomo for mayor.

Armstrong and her second-in-command, Veronica Turner-Biggs, will take the reins as the organized labor movement and the health care industry contend with the Trump administration’s attacks on collective bargaining rights and Congressional Republicans’ expected Medicaid cuts.

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