ENDA has a fascinating and long history. It's not the new kid on the block. I suppose one might say it has been around the block. A few times.
On May 14, 1974, the fifth anniversary of the Stonewall Rebellion, Representatives Bella Abzug and Ed Koch introduced HR 14752, the "Equality Act of 1974." The bill would have added "sexual orientation" to the 1964 U.S. Civil Rights Act, along with race, color, national origin, sex and religion.
That made a lot of people uneasy, not only because of the comparison to race, with its very different history, but also because the Civil Rights Act of 1964 made it illegal to discriminate in any context, including public accommodations, education, religious organizations, the military, etc. Many of us have been raised in an environment where race, gender and religious discrimination are considered immoral as well as illegal, unawares that this is but a recent development in our country's history. We are about 40 years out of the Dark Ages here, and the Middle Ages haven't happened quite just yet in most parts of the US.
In the early 90s, a new strategy emerged. Rather than trying to obtain all of the rights in the Civil Rights Act, the legislative efforts focused on employment rights, and the "Equality Act" was renamed the "Employment Non-Discrimination Act." In 1996, the bill came within one vote of passage, its success perhaps spurred by backlash from the recently passed DOMA, the "Defense of Marriage Act" that permitted states to ignore same sex marriages from other states. HRC sets out the timeline of ENDA introductions.
There's a lot of material out there on the web from past attempts at ENDA.
Some of this material is still interesting and useful. For example, prior polls show that most Americans are, in fact, in support of workplace protections for gay, lesbian, bisexual and transgender employees. A 2003 HRC slide show demonstrates that most Americans supported ENDA at that time. A 1997 American Psychological Association study backs up the understanding that being gay, lesbian, bisexual or transgender is not a "lifestyle," but an identity, and, makes a powerful argument that the "special rights" argument does not apply to a group subject to widespread prejudice. A 2001 study shows that reports of discrimination based on sexual orientation are roughly equal to those on race or gender. There are also studies showing that local anti-discrimination laws are ineffective, and federal law is needed. Cost estimates from the Congressional Budget Office from 2002 show that the EEOC estimated that their complaint caseload would rise by only 5 to 7%.
Regarding constitutionality, sponsors of the bill have explained that the Spending Power and Section Five of the Fourteenth Amendment give Congress the authority. The Spending Power provides the clearest authority for Congress to condition the receipt of Federal funding in programs and activities as defined by 42 U.S.C. 2000d-4a (2002). States that wish to obtain Federal funds for their programs or activities must comply with the reasonable, constitutional conditions placed on receipt of such funds. Further, Section Five of the Fourteenth Amendment clearly gives Congress the power to remedy sexual orientation discrimination in employment.
There's more out there. I've been adding some of this material to the Wikipedia article on ENDA, and I encourage others to contribute as well.