By Viktoria Dendrinou And Marcus Walker 

ATHENS--In Greece's July 5 referendum, as currently planned, voters will be asked to vote "no" or "yes" on a convoluted question about the country's creditors' conditions for further bailout aid.

How voters make sense of the ballot question could be decisive in determining the outcome. The referendum campaign so far is largely a contest to define the meaning of the question.

Prime Minister Alexis Tsipras and his left-wing Syriza party are seeking to convince Greeks that a "no" to creditors' proposals would safeguard national dignity and strengthen Athens's bargaining position for the next round of negotiations, without triggering an exit from the euro.

Opposition parties and foreign creditors, including the German government and the European Commission, are telling Greeks that they will be saying "yes" or "no" to Europe and its common currency.

The ballot paper is one of the more challenging that has ever been put to an electorate.

It reads: "Should the outline of the agreement submitted by the European Commission, the European Central Bank and the International Monetary Fund at the Eurogroup of 25/06/15 and is made up of two parts which constitute their unified proposal be accepted," before naming two documents written by those three institutions from bailout negotiations in Brussels that broke down last Friday.

The first document referred to on the ballot lists the fiscal austerity measures and other economic overhauls that creditors wanted Greece to enact under its EU-IMF bailout program, the European part of which expired at midnight on Tuesday. The second document is the institutions' "preliminary debt sustainability analysis," a regular feature of Greece's five-year bailout in which lenders try to predict Greece's debt trajectory assuming various scenarios.

The documents are replete with economic data and technocratic jargon. The government has posted Greek translations of the documents, originally written in English, online.

The creditors' proposals are strictly redundant, since they were supposed to be the basis for an agreement to extend Greece's previous bailout program beyond Tuesday. Since Tuesday has passed, the European program has lapsed. (The IMF's program continues until early 2016.) But European officials have said that any new bailout for Greece would involve substantially the same economic-policy and financing conditions.

Those conditions are the heart of the dispute between Mr. Tsipras's government and the lenders. Europe and the IMF want Greece to hit ambitious fiscal targets via a mix of spending cuts, including to pensions, and tax hikes. Athens wants to rely far more on tax increases while protecting pensioners. Greece--with support from the IMF--also wants more significant debt restructuring than other eurozone governments are willing to grant.

The consequences of a "yes" or "no" on July 5 are as murky as the question.

A majority for "yes," in the face of the Greek government's appeal for a "no," would probably lead to Mr. Tsipras's resignation, most analysts say. Mr. Tsipras himself has hinted that he might resign in such an event. It is unclear what would happen next. Fresh elections might be called. The parties in the current parliament might try to assemble a bipartisan coalition under a new prime minister. A new government might try to negotiate a new bailout deal with Europe.

Germany and other lenders would press the new government to sign up to their conditions quickly. Greece faces large repayments on bonds held by the ECB on July 20 and Aug. 20. Failure to pay those debts would make it very difficult, although not impossible, for the ECB to continue financing Greece's paralyzed banking system.

A "no" victory on Sunday would keep Mr. Tsipras in office for now, but could put Greece on a path toward default and exit from the euro. Mr. Tsipras claims it would help him to negotiate. Berlin and other lenders would probably be in no mood to start new negotiations.

Greeks will be able to vote between 7 a.m. and 7 p.m. on Sunday. Exit polls are expected to be published as soon as voting ends. The projected results are expected to become increasingly accurate as counting progresses. The world should have a good idea of the outcome within two or three hours of polls closing, unless the result is very close.

Costas Paris contributed to this article.

Write to Viktoria Dendrinou at viktoria.dendrinou@wsj.com and Marcus Walker at marcus.walker@wsj.com