NEW YORK (CNNMoney.com) -- Wal-Mart Stores is hitting back at some leading Democratic presidential hopefuls who have joined with the company's critics recently.

The world's largest retailer announced it is sending out what it terms a "voter education guide" to its 18,000 Iowa employees, criticizing politicians who have recently joined with the union-backed group "Wake Up Wal-Mart" for a series of rallies. Iowa holds the nation's first presidential caucus, and potential candidates are already visiting the state.

"We believe it's wrong for these political candidates to attack Wal-Mart and the transformation under way at our company," the letter said. "We would never suggest to you how to vote, but we have an obligation to tell you when politicians are saying something about your company that isn't true."

The company's critics charge that Wal-Mart fails to provide health care for more than half its workers, pays what it calls "poverty" wages, and helps to shift U.S. jobs overseas by selling low-priced imports.

The Wake-Up Wal-Mart group issued its own release in response to the Wal-Mart voter efforts.

"We welcome Wal-Mart's attempts to try and defend itself with its employees, political leaders and the American people," the group's statement said. "The debate over Wal-Mart is of critical importance to the future of our country. Sadly, not one single employee or any citizen will ever hear the truth from Wal-Mart about the negative impact it has on its workers or working families because Wal-Mart does everything it can to hide its real record."

Wal-Mart said it would invite the candidates to come to its Iowa stores and meet its employees. It released a letter it sent to one of the hopefuls, Sen. Evan Bayh, D-Indiana, from its regional manager in Iowa in which it said it wanted Bayh to visit "without the fanfare of media or staff or supporters.

"We don't want this to be about politics - the union-funded groups are already doing enough of that for the both of us," the letter said.

The company's letter to its Iowa employees said it is important that they help it respond to criticism.

"We want you to know that your voice matters when these political candidates attack your company. We urge you to talk with your friends, your family and your neighbors about the good Wal-Mart does," the letter said.

The company's press release said that polling data shows voters do not agree with attacks on Wal-Mart by candidates.

According to the release, 62 percent of those surveyed by Democratic pollster Thomas Riehle for the Cook Political Report disapprove of "Democratic candidates making Wal-Mart an issue in November's elections," while only 21 percent approve. Even a majority of union households opposed this strategy, according to the company.

Overall, 40 percent of registered voters would vote against an anti-Wal-Mart Democratic candidate, while just 18 percent would vote for such a candidate, the company's statement said.

Wal-Mart's letter to its employees says that published schedules show U.S. Sens. Evan Bayh of Indiana and Joseph Biden of Delaware, and Govs. Bill Richardson of New Mexico and Tom Vilsack of Iowa are taking part in the rallies against the company.

The company's letter does not mention Sen. Hillary Clinton of New York, another prospective candidate who the Wake-Up Wal-Mart site said has also joined the group in criticism of the company.

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