The risk is that the institution that has held the single currency area together through five years of debt crisis adopts a "QE-lite" plan that sends the wrong signal to markets, misses its target and jeopardizes the bank's credibility.
News that ECB President Mario Draghi met German Chancellor Angela Merkel privately last week highlights the acute political sensitivity of the decision. While the Bundesbank is opposed to the policy, which it sees as a back door to monetary financing of feckless governments, creating potential liabilities for German taxpayers, the ECB is hoping Merkel won't denounce it and will restrain her conservative allies.
Some QE supporters fear Draghi may make too many concessions to Germany by framing a program that is limited in volume and scope, doesn't share risk across borders and excludes buying bonds of countries with the lowest credit ratings.
Read MoreMarket to European Central Bank: Size of QE matters!
A draft plan circulated by ECB staff this month would cap purchases at 500 billion euros. Some market analysts say it may need to be twice that size or open-ended to reach its objective.
Draghi has a majority on the policy-making Governing Council but the two German members, Bundesbank president Jens Weidmann and ECB executive board member Sabine Lautenschlaeger, seem sure to vote against QE. A handful of other governors may join them.
One option seriously under consideration, ECB sources say, is to make each national central bank in the 19-nation Eurosystem shoulder the risk of default on all or most of its own country's bonds that are purchased.
"This would be at best ineffective and at worst dangerous," said Guntram Wolff, director of the Bruegel economic think-tank in Brussels. "(It) would be a strong signal that the ECB is no longer a 'joint and several' institution."
Veteran Danish economist Niels Thygesen, one of the experts who designed original blueprint for the euro in the 1980s, told Reuters: "If these are the circumstances for some QE, it may not be worthwhile even for strong enthusiasts."
It would be a limited program designed by lawyers as much as economists.
Yet by declaring ECB bond-buying legal and urging courts to be cautious in challenging the central bank when they lack the expertise, the European Court of Justice's advocate general last week effectively gave a green light for full-blooded action.
Although his opinion concerned a lawsuit brought by German Eurosceptics against a previous, as yet unused bond-buying scheme, it had a clear read-across to QE.