The Supreme Court on Thursday opened wide the gates to allow more corporate and union money to finance political campaigns – and potentially influence politicians and lawmaking.

That’s unfortunate, and means that the role of watchdogs tracking the money trail will be more important than ever.

It’s not as if corporations and unions have so far had their wallets glued shut. They can fund issue ads that are important to their interests. And they’re allowed to form political action committees that directly support candidates, as long as the donations are collected voluntarily from employees and union members.

But even members of Congress, whose energy is increasingly diverted to fundraising, have long recognized the potentially corrupting effect that big money can have on them. More than 100 years ago they banned corporations from donating directly to federal candidates.

Thankfully, the justices upheld that ban Thursday, as well as disclosure rules about contributors. But in a divisive 5-to-4 ruling, they overturned other important restrictions.

In time for this year’s midterm elections, corporations and unions can now spend directly from their treasuries on ads to support or defeat candidates – as long as those ads are produced independently and not coordinated with a campaign. They may also run ads right up until election day, instead of pulling them 30 days before a primary and 60 days before a general election.

Writing for the majority, Justice Anthony Kennedy grounded the ruling in First Amendment rights. Corporations and unions – like individuals – have a right to free speech, the majority reasoned. “The censorship we now confront is vast in its reach,” he wrote.

But Justice John Paul Stevens said in his dissent, “The court’s ruling threatens to undermine the integrity of elected institutions around the nation.” Indeed, when voters say they want “change” in Washington, the influence of money on politics is the kind of thing they’re talking about.

The high court has the final word, but not the only word. Groups that track campaign fundraising will need to raise their voices when they find wrongdoing or worrisome signs – even as they have more money to watch. Those groups include nonprofits; the news media; and the official watchdog, the Federal Election Commission.

Yet the traditional media are downsizing and the FEC, which has an equal number of Republican and Democratic commissioners, has a poor track record. The FEC could be strengthened if its members were recommended to the president not by the Senate, which has a vested interest in a watchdog with no bark, but by an independent commission.

Transparency is now more important than ever, and so is beefing up the competition to corporate and union donations – small donors and public campaign financing. The Internet has given small donors a much bigger voice, but that can be amplified even further by Congress.

Now is the time for lawmakers to pay serious attention to proposed legislation that would allow House and Senate candidates to choose to run for office without relying on large contributions, big money bundlers, or donations from lobbyists. If candidates commit to raising a certain amount of small donations, they can qualify for competitive public funds for their campaigns.

Thus, candidates would divert far less energy to fundraising – which can take as much as 40 to 50 percent of the time of members of the House, who face election every two years. And, candidates would be less beholden to monied special interests.

The Supreme Court has significantly altered the landscape of campaign finance. The next election will reveal how the new arrangement works in practice.

Watchdogs must keep their eyes open.