All elected officials, and those who help finance elections in the expectation that certain promises will be kept — and everyone who cares about the rule of law — should hope the Supreme Court agrees to hear Don Siegelman’s appeal of his conviction. Until the court clarifies what constitutes quid pro quo political corruption, Americans engage in politics at their peril because prosecutors have dangerous discretion to criminalize politics.

Siegelman, a Democrat, was elected Alabama’s governor in 1998 and was defeated in 2002. In 2006, he and a prominent Alabama businessman — Richard Scrushy, former chief executive of HealthSouth — were convicted of bribery. Here is why:

As governor, Siegelman advanced a ballot referendum to create a state lottery with revenue dedicated to education. After Scrushy raised and contributed $250,000 to the lottery campaign, Siegelman appointed him — as three previous governors had done — to a health-care-related state board, three members of which are required by law to be health-care professionals and all members of which serve without pay.

The lottery referendum lost, leaving the Democratic Party with a campaign debt. Siegelman and a wealthy Alabamian guaranteed a loan to pay it off, but Siegelman and others raised sufficient money — including another $250,000 from Scrushy through his company — to retire it. Note that half of the contributions involving Scrushy came after his appointment to the board.

Now, it is not unreasonable to suppose that, before Scrushy’s first contribution, he hoped to ingratiate himself with the governor, that he hoped to be reappointed to the board and that Siegelman appointed him at least partly because of gratitude. But was this bribery?

A jury said yes, and Siegelman was sentenced to seven years in prison. He was released after nine months, pending his appeal, which turns on the contention that due process is denied when the law does not give due notice of proscribed behavior. It is this due notice that circumscribes prosecutorial discretion.

Siegelman argues that political contributions enjoy First Amendment protection, and seeking them is not optional for a politician in America’s privately funded democracy. Furthermore, elected officials must undertake official acts; some will be pleasing or otherwise beneficial to contributors. (See Solyndra.) Often this is nothing more than keeping campaign promises: People contribute because they endorse a candidate’s agenda.

One circuit court (Judge Sonia Sotomayor writing) has held that to establish bribery involving political contributions requires proof of an “explicit” quid pro quo, meaning “an express promise.” Indeed, the Oxford English Dictionary defines “explicit” as “stated clearly and in detail, leaving no room for confusion or doubt.”

Another circuit court, however, has held that “explicit” does not mean an “express” or actually and clearly stated promise that an official action will be controlled by a contribution. Rather, “explicit” quid pro quo can mean only a state of mind inferred from perhaps suspicious circumstances.

But if bribery can be discerned in a somehow implicit connection between a contribution and an official action, prosecutorial discretion will be vast. And there will be the political temptation to ascribe unspoken but criminal mental states to elected officials. The Supreme Court can circumscribe this dangerous discretion by affirming the principle that the quid pro quo standard for bribery requires proof, not a mere inference, of an actual communication. In the law’s current, contradictory condition, the line is blurry between the exercise of constitutional rights and the commission of a crime.

Politics in a democracy is transactional. Candidates routinely solicit the support of interest groups, from unions to business organizations to environmental and other advocacy factions. (“If you vote for me, I will do X for you.”) And many such groups solicit the solicitousness of candidates. (“If we support you, will you do Y for us?”)

It is not uncommon for wealthy individuals to support presidential candidates lavishly, if not for the purpose of becoming ambassadors, then with the hope that the president-elect will show gratitude for their generosity. The Washington Post of Jan. 19, 2011, reported (“Embassy openings for open wallets”) that President Obama’s political appointees, as opposed to career diplomats, had received 30.05 percent of ambassadorial posts, just below the 30.47 average of the previous five presidents.

In 2009, a bipartisan amicus brief by 91 former state attorneys general urged the Supreme Court to use Siegelman’s case to enunciate a clear standard for establishing quid pro quo bribery. Today’s confusion and the resulting prosecutorial discretion chill the exercise of constitutional rights of political participation and can imprison people unjustly.

georgewill@washpost.com