Some predict that the Supreme Court's recent Citizens United ruling will radically redefine American politics, while others argue that its impact will be negligible.

But the truth is, no one really knows how the ruling will play out in the real world of elections -- least of all the corporations, unions and other political players gearing up to spend heavily in this year's high-stakes midterms.

Few dispute that the ruling, which ended the ban on direct political spending by corporations and (by extension) unions, will step up political activity by non-party interest groups. These include nonprofit advocacy groups, trade associations and so-called 527 organizations.

(Sen. Charles Schumer, D-N.Y. has announced a hearing to take place on Feb. 2 on legislation to blunt the impact of the Citizens United ruling.)

Corporations, in particular, may resist spending directly on campaigns to avoid alienating customers, they now enjoy unprecedented freedom to underwrite the associations and surrogates that represent them.

But which groups will flourish most under the new regime, and which will face constraints? What types of spending will be permitted, and how much must be disclosed? These and other questions are already consuming election lawyers around town and putting pressure on the Federal Election Commission to clarify muddy areas of the law.

The biggest unanswered question is what defines coordination between a corporation, union or other political player and a candidate. In Citizens United v. FEC, the high court lifted only the restrictions on so-called independent expenditures -- that is, the political money corporations may spend when they are not coordinating their activities with specific candidates.

The distinction looks straightforward on the surface, but it's one that's long stymied Congress and the FEC. Coordination might mean direct talks between candidates and political players, for example. Or it may mean sharing political consultants, or ad messages and talking points.

With the 2002 Bipartisan Campaign Reform Act, Congress ordered the FEC to come up with a better definition of coordination. But the agency foundered, and failed repeatedly to come up with regulations that stood the test of court challenges by the law's authors.

The Citizens United ruling frees up corporations to participate directly in elections, "but they're going to have to do so only independently," said Trevor Potter, general counsel of the Campaign Legal Center and a former FEC commissioner. "And I think that's going to be a major battleground going forward. What is coordinated? What is independent?"

"It's not an easy issue," concurred election lawyer Lawrence Noble, counsel at Skadden, Arps, Slate, Meagher & Flom. "And if the FEC writes regulations that are very narrow in terms of coordination, then you've really opened up the door to corporations working with candidates. If you make it too broad, you run the risk of making it unconstitutional."

Lawmakers on Capitol Hill have pledged to step in shortly to clarify these and other questions, and to impose new constraints on corporations. Schumer, who chairs the Senate Rules Committee, along with Rep. Chris Van Hollen, D-Md., are drafting a measure that would better define what constitutes coordination between outside political groups and candidates. Under discussion are limits on sharing advertising materials or consultants, and on communication between outside groups and candidates.

"Those are some of the big weaknesses that we now have that need to be closed," said Craig Holman, legislative representative for Public Citizen.

Schumer and Van Hollen are also considering new disclosure rules and shareholder protections. The latter could ban CEOs from making political expenditures from corporate treasuries without shareholders' approval.

In the long term, reform advocates are pushing hard for public financing for both congressional and presidential candidates. A congressional public financing bill now has close to 130 co-sponsors in the House, and its backers argue that momentum is building. There's also talk of a constitutional amendment that would allow Congress to ban corporate political spending.

Such ambitious proposals, of course, will be tough and time-consuming to enact. In the meantime, newly empowered political players are still wondering what, exactly, the Citizens United ruling means for them. Nonprofits such as 501(c) 3 charities, and 501(c) 4 political advocacy groups, in particular, may have to tread carefully. Citizens United or no, the IRS still bans partisan political activities for charities, and limits it for advocacy organizations.

Such groups "still have to be concerned about the IRS," said Noble. "So I can see some group out there suddenly thinking all constraints are off, and then running into trouble with the IRS."

Another gray area, Noble added, is which rules apply to a group that becomes so active in elections that politics becomes its major purpose. Such a group may then meet the definition of a political committee under FEC rules, meaning that it could no longer accept direct corporate or union contributions.

It's just one of many areas that the revolutionary Citizens United ruling has thrown open to interpretation. As Noble put it: "There are numerous questions like this. And that's what makes these opinions so interesting -- and difficult."