The Washington Post explained the politics. It looks like some highly-placed people in both political parties haven't discovered that the Cold War is over and our side won. Instead of helping America to move on to meet new challenges, they want to keep the Cold War going by perpetuating its structures. They are committed to re-fighting the last war instead of preparing for the future. NATO was the linchpin of the Cold War strategy to scare the old Soviet Union out of invading Western Europe.
Nobody is worried any longer about Soviet troops marching into Western Europe.
The NATO Expansion Treaty would be a profound change in that mission. All those borders were established as a result of bloody battles. There is nothing sacred about them, and there is no reason why America should promise to go to war to preserve them. This dramatic expansion of our commitments to expend our blood and treasure has had almost no media coverage outside the Beltway, so the American people are almost wholly in the dark about it. It is irresponsible for the Republican Senate to ratify this watershed commitment without a national debate.