WHITE HOUSE press secretary Jay Carney spent a good chunk of his time during a briefing last week trying to explain why President Obama has declined to issue an executive order barring federal contractors from discriminating on the basis of sexual orientation.

Mr. Carney struggled for some eight minutes but was unable to give a satisfactory answer. That’s understandable, because there is no principled reason for refusing to extend such workplace protections to millions of Americans.

Since the administration of President Lyndon B. Johnson, federal contractors have been prohibited from discriminating on the basis of race, national origin, religion and sex. This nondiscrimination policy was promulgated through an executive order and has been upheld numerous times against legal challenge. Mr. Obama could amend the existing order to include sexual orientation or could craft a separate order to prohibit those who wish to or already do business with the federal government from discriminating against lesbian, gay, bisexual or transgender (LGBT) employees.

Such an order would have a beneficial and immediate impact on a large swath of the U.S. workforce: Federal contractors employ some 26 million people. Although many — especially the largest — already have nondiscrimination policies that include sexual orientation, some 16.5 million of these employees work for companies that do not, according to the Williams Institute. A recent poll of likely 2012 voters showed that nearly three-quarters of respondents favored such protections for LGBT employees.

The president, Mr. Carney argued, “is committed to lasting and comprehensive non-discrimination protections,” but those protections would be best achieved through a “legislative solution,” such as the Employment Non-Discrimination Act (ENDA). ENDA, which would extend prohibitions against sexual orientation discrimination to all but the smallest private-sector employers, is a worthy piece of legislation, but its passage appears remote and likely will remain so if conservatives control either chamber of Congress or if the GOP candidate captures the White House in November.

Focusing on the grand gains achieved through legislation also ignores the important role played by executive orders and other administrative acts in paving the way for change. Mr. Carney, for example, pointed to the 2010 repeal of the “don’t ask, don’t tell” policy as a model of legislative achievement, but he failed to note that independent presidential action preceded the repeal. Before Congress voted to ditch the wrongheaded policy that prevented openly gay men or lesbians from serving in the military, the president took steps on his own to require, for example, review by higher-level officers before a service member could be ousted.

The president played a pivotal role in the repeal of “don’t ask, don’t tell” and also deserves credit for refusing to defend the discriminatory Defense of Marriage Act. He should again seize the mantle of leadership by issuing an executive order that prohibits the federal government from doing business with contractors that fail to guarantee basic fairness to their LGBT employees.