Oliver Koppell, a former city councilman and state attorney general, is set to launch a primary challenge to Independent Democratic Conference Leader Jeff Klein, a source familiar with the plan said.

A formal announcement is expected on Monday.

The primary battle has long been hinted at by Koppell, who has become increasingly critical of Klein in recent weeks.

Nevertheless, toppling Klein is not an easy lift.

The Bronx lawmaker is a prodigious fundraiser and has the backing of the Bronx Democratic political establishment.

Liberal advocates hope, however, that a primary challenge could provide some added pressure to Klein, and possibly the state Senate, to pass a statewide version of the public financing of political campaigns.

Klein backs public financing, but the measure remains firmly opposed by Senate Republicans, who are in a governing coalition in the chamber alongside Klein’s IDC.