WASHINGTON — The National Education Association, the nation’s largest union, endorsed Democrat John Kerry for president Monday, the final touch in its campaign to drive up school spending and reshape the biggest education law in decades.

The NEA, a 2.7-million member group composed mainly of teachers, is out to advance its agenda on everything from testing students to halting private-school vouchers. The union is mobilizing its money and forces for Kerry — targeting political staff in 15 swing states, going into schools to rally its members, and joining liberal groups to organize a massive night of political house parties.

Kerry, who is scheduled to speak to the 9,000 delegates at the NEA convention today, was endorsed by 86.5 percent of them. The Massachusetts senator offers many teacher-friendly promises the union likes, but he also advances ideas the NEA has long opposed, such as paying bonuses to teachers based on student test scores.

Although a quarter of NEA members identify themselves with the Republican Party, the union has never endorsed a Republican for president and typically spends $9 out of every $10 it raises on Democrats. Its relationship with the Bush administration has been particularly prickly since his education secretary, Rod Paige, jokingly referred to the union as a “terrorist organization” and even annoyed members with how he apologized for the remark.

To the public, education is behind the Iraq war, terrorism and jobs as a voting priority. But it could make the difference among key groups, such as Hispanics and married women with children, political strategists say.

At the heart of the political debate in education is No Child Left Behind, the law championed by Bush and supported by Kerry in 2001. It requires a highly qualified teacher in all core classes, expanded standardized testing, more choices for parents and public reporting of how well all major groups of students perform. Schools that get federal poverty money but don’t make enough progress face penalties.

The union opposes several aspects of the law, from the special rules given to teachers in alternative training programs to federal spending that, while at record levels, falls well short of the maximum allowed.