It’s never too early to run for mayor.

Consider City Comptroller Bill Thompson – a principal custodian of, and adviser to, the city’s five public-employee pension funds.

The funds’ combined portfolio is some $105 billion – big enough to matter – and last week, Thompson announced that two dozen companies in which the funds are invested must bar hiring discrimination based on sexual orientation and “gender identity.”

A press release characterized this move as an “unprecedented effort to push corporate America to protect LGBT [lesbian, gay, bisexual, transgender] workers.”

Well, it is nothing if not that.

And it is also totally out of bounds.

Until recently, Thompson has fulfilled his role as a funds trustee responsibly.

This contrasted significantly with, say, former state Comptroller Carl McCall – who had a history of using his position as the sole trustee of state pension funds to bully companies into adopting politically desirable goals. (In one notorious example, McCall tried to blackmail pharmaceutical companies to lower prescription-drug prices.)

However, as the 2009 mayoral election looms, leading contender Thompson has become – political.

The five city pension funds hold nearly 8 million shares of WalMart stock; last summer, Thompson demanded that the company turn over records so he could determine whether it had been spying on shareholders.

The inquiry was purely political – WalMart being a prime target of the city’s liberal establishment.

And now, last week’s action.

A private company’s hiring practices are, arguably, fair game for comment by a mayoral candidate. But a comptroller has no legitimate business using the funds as a weapon.

Thompson needs to stop it.