By Jeff Mason and Lucia Mutikani

BOSTON/WASHINGTON (Reuters) - President Barack Obama on Monday ordered government contractors to offer their workers seven days of paid sick leave a year and, without naming them, knocked Republican presidential candidates for advocating what he said were anti-union policies.

Obama signed an executive order on sick leave, which the White House said would affect some 300,000 people, during a flight to Boston, where he spoke at a union event.

Starting in 2017, workers on government contracts will earn a minimum of one hour of paid sick leave for every 30 hours worked. Contractors can offer more generous amounts at their discretion.

Speaking to a friendly crowd without a tie or jacket, Obama said such policies were beneficial to employers and said more worker friendly measures, such as paid maternity leave, were needed.

"Right now, we are the only advanced nation on Earth that does not guarantee paid maternity leave," he said.

"Now, for the men in the audience in particular, think about that. We wouldn’t even go to work if we had to carry around somebody for nine months. The human race would evaporate," he said, drawing laughter and applause from the crowd.

Unions and organized labor are a key constituent for the Democratic Party, and their support will be critical in the 2016 presidential election.

Obama, who joked that he was glad not to be on the ballot next year, made thinly veiled references to Wisconsin Governor Scott Walker and New Jersey Governor Chris Christie for anti-union remarks and policies. He did not name them by name.

"It's clear he stands with the big government union bosses while we stand with the people," Walker, during a campaign stop in New Hampshire, said in response to the president.

The executive order follows a series of measures by the White House to expand access to paid leave. In January, Obama issued a presidential memorandum directing the government to advance up to six weeks of paid sick leave for the birth or adoption of a child, or for other sick leave-eligible uses.

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Obama is also pressing Congress to pass legislation giving government employees six additional weeks of paid parental leave. Labor Secretary Thomas Perez said he could not say what the cost of implementing the seven-day paid leave rule would be to contractors.

"We believe the cost of implementing this rule is offset by the efficiencies that come with reduced attrition, increased loyalty, all of those things that have been documented in a number of studies of state laws that have been enacted," Perez told reporters on a conference call on Sunday.

Obama also used the trip to Boston to renew his call for Congress to pass the Healthy Families Act, which would require all businesses with 15 or more employees to offer up to seven paid sick days each year.

According to the White House, an estimated 44 million private-sector workers, about 40 percent of the total private-sector workforce, do not have access to paid sick leave.





(Editing by Paul Simao and Cynthia Osterman)