President Obama on June 13. (Pablo Martinez Monsivais/AP)

President Obama will soon sign an executive order banning federal contractors from discriminating against gay men, lesbians and others on the basis of sexual orientation, the White House said Monday.

The move is the last significant action Obama is likely to be able to take to advance gay rights without the cooperation of Congress, activists said.

Once signed, the executive action will have an impact on some of the biggest companies that do business with the government, including Exxon Mobil and Tysons Corner-based SAIC.

Obama made the decision to sign the executive order after years of pressure from gay-rights groups, which frequently complained that he had dawdled on the issue after promising to take action during the 2008 campaign.

Coming a day before Obama is scheduled to address gay-rights advocates at a New York fundraiser, the move could help rally the Democratic base in an election year when voter turnout will be critical. A number of Democratic candidates rushed to declare their support for the measure Monday.

“Sometimes the gears of government turn slowly, but I’m very glad they have turned,” said Sen. Jeff Merkley (D-Ore.), who faces a tough reelection fight this fall against GOP candidate Monica Wehby and who sponsored legislation on the issue in the Senate.

Many gay-rights activists described the executive order as the fourth and final step that Obama can take as president to expand protections for gay, bisexual and transgender Americans.

The move follows a 2009 hate-crime law and 2010 legislation that repealed the “don’t ask, don’t tell” policy, which barred gays from openly serving in the military. The administration is also in the midst of an aggressive effort to extend benefits to gay couples following a Supreme Court decision striking down part of a 1996 law that defined marriage being solely between a man and a woman.

In a private meeting with activists Thursday, White House officials are expected to provide more detailed guidance on how they plan to work toward making sure gay spouses receive as many benefits as possible. But they are also expected to describe where legislation is needed to ensure that gays can receive Social Security or veterans benefits through their spouses.

“By issuing an executive order prohibiting federal contractors from discriminating against LGBT people, the president will not only create fairer workplaces across the country, he will demonstrate to Congress that adopting federal employment protections for LGBT people is good policy and good for business,” said Chad Griffin, a former Obama donor who is president of the Human Rights Campaign advocacy group.

In 2012 — not long before Obama announced his support for same-sex marriage — senior White House officials, including Obama’s close adviser Valerie Jarrett, called gay-rights advocates into a meeting and told them the president would not be signing an executive order expanding workplace protections at that time.

Advocates released statements at the time blasting the administration. Obama’s subsequent endorsement of gay marriage quieted the concerns for a time. But activists began their push anew last fall after Republicans in the House said they would not follow the Senate in passing the Employment Non-Discrimination Act, or ENDA, which bans workplace discrimination based on sexual orientation and gender identity.

Several Democratic donors refrained from making donations to the party’s three campaign committees this cycle out of frustration with the administration’s refusal to act, according to a fundraiser who asked not to be identified in order to discuss internal deliberations.

The decision to pursue the executive action appears to reflect a calculation by Obama that passing ENDA is unlikely before his second term ends.

Top GOP leaders — including Senate Minority Leader Mitch McConnell (Ky.) and House Speaker John A. Boehner (Ohio) — declined to comment on the announcement Monday.

But conservative advocacy groups criticized the decision. Peter Sprigg, a senior fellow for policy studies at the Family Research Council, said it “will give activists a license to challenge their employers whenever they feel aggrieved, exposing those employers to threats of costly legal proceedings and the potential of jeopardizing future contracts.”

The White House did not say when Obama will sign the executive action, but activists speculated it could come later this month at a gay-rights event at the White House. The text of the order could be influenced by a Supreme Court decision on the Hobby Lobby case, which centers on whether a private employer can be required to make birth control coverage available for its employees despite religious objections.

Rose Saxe, a senior staff attorney at the American Civil Liberties Union, said any decision in that case “might certainly embolden demands for a broader exemption” for religious contractors. But such employers make up a tiny fraction of federal contractors, and Saxe questioned whether they would obtain such an exception.

National Economic Council Director Jeffrey Zients said the White House spent time engaging “with a broad range of parties to better understand the cost of discrimination” against LGBT employees and concluded that the change would produce economic benefits.

“Workplace equality is not just the right thing to do, it’s also good for business,” Zients told reporters on a conference call.

A 2013 survey of LGBT adults by the Pew Research Center found that 21 percent reported facing workplace discrimination.

The Williams Institute at the University of California at Los Angeles, a research organization for LGBT issues, says 43 of the top 50 federal contractors include sexual orientation in their nondiscrimination policies and about half include gender identity. The largest firms, including Lockheed Martin, Boeing and Raytheon, all include sexual orientation and gender identity in their nondiscrimination policies.

Advocates have targeted Exxon Mobil because it does not specify sexual orientation and gender identity in its overall nondiscrimination policy. Exxon does hundreds of millions of dollars a year in business with the federal government.

“The most immediate practical impact of this executive action is that Exxon Mobil’s corporate management now faces a decision about whether they will finally amend their policies or forfeit the opportunity to profit from hundred of millions of dollars in taxpayer contracts,” said Tico Almeida, president of Freedom to Work, an organization that has filed a complaint against Exxon for discriminatory policies.

Scott Silvestri, a spokesman for Exxon, said the company “strictly prohibits any form of discrimination by or toward employees, contractors, suppliers and customers in any ExxonMobil workplace.”