More than 300 companies—including 20 of the nation’s 50 largest, such as Apple, Wal-Mart and General Motors—have agreed to reassess their hiring practices at President Obama’s request to make sure they are not biased against Americans who have been out of work for more than six months.

Obama is gathering some of those business leaders at the White House Friday who have pledged to make a concerted effort to hire the long-term unemployed, part of his ongoing campaign to mobilize action outside of Washington to achieve his policy goals.

Gene Sperling, who directs the White House National Economic Council, told reporters in a conference call Thursday that the administration started reaching out to corporate leaders in May and secured specific pledges from companies over the past three to four months.

“This was an area where we should be able to move the needle by talking to companies directly and getting them to examine their business practices,” he said. “The response has been inspiring.”

“What we have done is to gather together 300 companies, just to start with, including, some of the top 50 companies in the country, companies like WalMart and Apple, Ford and others, to say, Let’s establish best practices,’ ” Obama said in an interview with CNN on Thursday. “Because they’ve been unemployed . . . so long, folks are looking at that gap in the résumé and they’re weeding them out before these folks even get a chance for an interview.”

Several academic studies suggest that the very fact of being unemployed for an extended period of time can reduce one’s chances of getting hired. One recent study showed those who had been unemployed for eight months had a 45 percent lower interview callback rate than those out of work for one month. A separate survey reported those unemployed for seven months need to send an average of 35 resumes to online job postings to receive just one interview, compared to just 10 resumes per interview for those unemployed for only one month.

“We are trying to address the heart of that negative cycle,” Sperling said.

In all other respects, he added, “the long-term unemployed tend to look like the rest of us.” Seventy percent of them are under 50, he noted, and they are “slightly more educated” than the average person who is seeking a job.

Top Obama aides, including Sperling and White House senior adviser Valerie Jarrett, made a personalized pitch to CEOs at gatherings inside and outside the White House. At one point Sperling e-mailed media titan Rupert Murdoch to ask if he would consider taking the pledge; Sperling said Murdoch e-mailed him back and agreed to scrutinize his companies’ hiring practices. Both News Corp. and 20th Century Fox eventually signed onto the effort.

“We still consider this not a destination, but a launch,” Sperling said. “”I have no question that after tomorrow, there will be more companies that will come to sign up.”

Obama will also announce Friday a $150 million grant competition through the Labor Department to support public-private partnerships geared toward helping prepare and place the long-term unemployed in jobs.

Sperling said he could not estimate how many people the new commitments would affect, but he predicted more people would become employed as a result.

It “will absolutely be a net win for the country. This will not be a zero-sum game,” he said.