The alliance of the National Education Association and American Federation of Teachers would create a 3.3 million-member union with enormous political clout at a time when public education has been pushed to the top of the national agenda.
The deal needs approval from both groups' general membership at national conventions in July, and even if passed would not be final until 2002. However, the AFT and NEA already have created a joint leadership council to work on public policy issues, including teacher quality, school safety and discipline and school building improvements.
The larger of the two, the NEA, said it now could push these issues more aggressively, safe from the fear of alienating members who might jump ship to the AFT.
"We're stepping out. We're doing things differently and we're taking risks. And if you're going to take a risk, the last thing you want is a competitor examining every move you make and using it as an organising opportunity," NEA spokesperson Kathleen Lyons said.
Negotiations have been under way for four years and in 1996 the unions reached a "no-raid" agreement, promising not to recruit each other's members.
That left the NEA's Representative Assembly feeling safe enough last year to accede to controversial peer reviews of teacher performance - something many of its older members had opposed. "That was heresy just a couple of years ago," Ms Lyons said.
Together, the two unions will have even more political clout. Already they are pushing for $30 billion in federal spending for schools, an appeal echoed by President Clinton when he made education the major theme of his State of the Union address last month.
The AFT has long been the more conventional-style labor union, fighting over working conditions and contracts since its founding, while the NEA began as a professional association that did not even call itself a union until the 1960s. It has focused on the more cerebral issues of education policy.
"We have traditionally been adversaries," Ms Lyons said. "We never used to even talk to each other. It got pretty ugly."
"When problems come up with urban schools, the NEA has really not been much of a voice at the table and we believe we have a lot of expertise we can bring to bear if we have more of a legitimate standing."