President Obama is mulling an executive order to force big government contractors to disclose details of their political spending. Big businesses are already telling their political patrons in Congress to oppose it -- and the pressure is building.

The president should issue the executive order immediately. And he should go even further by banning all political activity by companies receiving more than half their revenues from the U.S. government.

Lockheed Martin, the nation's largest contractor, has already got more than $19 billion in federal contracts so far this year. But we know very little about Lockheed Martin's political spending other than its Political Action Committee contributions. We don't know how much money it gives to the Aerospace Industries Association to lobby for a bigger defense budget.

We don't even know how much Lockheed is giving the U.S. Chamber of Commerce to lobby against Obama's proposed executive order requiring disclosure of its political activities.

Don't we have a right to know? After all, you and I and other taxpayers are Lockheed's biggest customer. As such, we're financing some of its lobbying and political activities.

Lockheed's lobbying and political activities are built into its cost structure. So when Lockheed contracts with the federal government for a piece of military equipment, you and I end up paying for a portion of its political costs.

It's one of the most insidious conflicts of interest in American politics.

Now, in the wake of the grotesque Supreme Court decision, Citizens United vs. the Federal Election Commission, there's no limit on what Lockheed can spend on politics.

That's why the president should go the next step and ban Lockheed and all other government contractors that get more than half their revenues from government from engaging in any political activities at all.

Otherwise, you and I and other taxpayers indirectly pay for Lockheed and Northrop Grumman to lobby for a larger military budget and support politicians who will vote for it.

We indirectly pay for Blackwater to lobby for -- and support politicians who will demand -- more use of contract workers in Iraq and Afghanistan.

We indirectly pay for Raytheon and General Dynamics to lobby for, and support politicians who will push for, more high-tech weapons systems.

And so on.

Disclosure is a start. But in this post-Citizens United world, it's only a beginning of what's needed.

Robert Reich is the author of Aftershock: The Next Economy and America's Future, now in bookstores. This post originally appeared at RobertReich.org.