If there ever was an appropriate target for a big, fat lawsuit, it’s New York City’s newly revamped public campaign-finance system.

Happily, 13 plaintiffs filed just such a suit in Manhattan Federal Court Monday.

While the Campaign Finance Board is the named defendant, the target is a law passed last year by the City Council and signed by Mayor Bloomberg. It banned contributions from limited-liability corporations and partnerships and severely limited donations from virtually anyone – lobbyists, developers, contractors, etc. – who does substantial business with the city.

The law reduced the cap on contributions from those sources from $4,950 to $400 for mayoral races and $2,950 to $250 for City Council contests.

More significantly, the statute explicitly exempted labor unions – organizations that, arguably, do the greatest amount of business with the city (most of it of the monkey variety).

In other words, the law OKs a union raising and spending virtually unlimited amounts of money to help elect a mayor or City Council members – but hobbles everybody else.

It’s hardly a surprise that the unions were exempted. After all, how would council members get elected if not for union campaign cash?

Consider:

* Of the top 10 donors to City Council candidates in 2005, seven were labor unions, with New York’s health-cartel behemoth, Local 1199, kicking in more than $100,000.

* Of the top 10 donors to candidates participating in the city’s campaign-finance system, eight were labor unions – 1199 again ranking No. 1, with more than $140,000.

And those figures don’t even take into account the value politicians get from the unions’ get-out-the-vote operations.

The law also is obnoxious – and vulnerable – on First Amendment grounds. The US Supreme Court has ruled several times that overly severe campaign-contribution limits are unconstitutional.

So good luck to the plaintiffs.