The Supreme Court’s recent decision to allow corporations and unions to directly support political candidates through advertising has unleashed a frenzy of speculation about the mass infusion of cash into political races beginning with the midterm elections of 2010. I think the reality may be a little less frenzied than everyone fears.

The court’s decision undoubtedly provides a huge new weapon in the political arsenal for the warring factions of liberalism and conservatism. But, I rather suspect that as in the good old days of mutually assured destruction – when U.S. and Soviet missile arsenals guaranteed that neither side would launch a nuclear war – the weapons of mass advertising may not be used in the way that many commentators now expect.

Given how fashionable corporate bashing has become of late, no doubt many businesses and unions will be tempted to take advantage of the Supreme Court decision by directly engaging in the kind of political advertising that has heretofore been prohibited.

As we all know, much time, effort and advertising dollars are currently spent on advocacy advertising, advancing companies’ interests on a range of public policy issues. (My firm is among the many that produce such ads.) So, it would only be natural for the companies to take the next logical step from advocating about issues to advocating directly for the election or defeat of those politicians who ultimately pass or defeat federal legislation.

But, I would caution against their doing so – and as a lifelong Democrat I’d urge the restraint on unions as well. The reason is simple. I think the results will be counterproductive.

A public that already believes that its elected leaders are beholden to special interests of the left and right, are only going to have that feeling exponentially magnified when they see that the Acme Explosives Company is running ads in favor of Senator Bob Jones and the Amalgamated Explosives’ Workers Union wants the voters to send Representative Sue Smith to Washington.

And at least as important as what the public feels, imagine how the office-seekers themselves are going to view the offer to have corporate or union interests come out so publicly in support of their candidacy. Will Senator Jones and Representative Smith be eager to be visibly known as Acme’s or Amalgamated’s candidate? I would think not.

The power of advertising cannot be minimized in a political environment, but there is another reason that the candidates and their supporters may be less inclined to fire off all their weapons. Every ad will produce a barrage of coverage in the media and the blogosphere, attacking the advertisers and diluting – if not downright undermining – the message that was bought and paid for.

Activists in each political party may be the people who drive the nominating process and are essential to making a candidate viable in the general election campaign. But the winner of the general election is usually – if not always – the individual who appeals to the greatest number of independent voters. And I find it hard to believe that independent voters will be persuaded by direct advertising from either businesses or unions.

American corporations and unions can, should and do actively participate in our political process by public relations work, direct lobbying and the somewhat arms-length advertising that was the law of the land as of last week.

But I’m hard-pressed to see how major companies will enhance their standing with shareholders and stake holders by spending large amounts of their money on direct support of candidates, when they should be trying to grow the bottom line in tough times. Spending large sums of money on campaign commercials would likely alienate investors, customers and other public officials.

So while I am counseling restraint, I am not at all sure that circumstances will make this the most practical advice. One lobbyist said today that lobbyists can tell law-makers, “We have got a million dollars we can spend advertising for you or against you – whichever one you want.”

Clearly this is not in the public interest for this to happen, but it may well be the result. If one side decides to do what the Supreme Court now allows and fires off its missiles, the whole democratic process could change in fundamental ways that we cannot even begin to imagine, and in ways that will likely not be in the public interest.

This is one arms race both sides would do well to avoid.

Harris Diamond is CEO of Weber Schandwick and CEO for the Constituency Management Group of the Interpublic Group of Companies.