the answer is within your post, so you most likely are not as dumb as you might think, with regard here.
they want to deny Kavanaugh because he is viewed as a stauch conservative and if he is put onto the Court, he will heavily tip the scales with a conservative slant. if they can deny him or postpone the confirmation hearing until after the mid terms, by winning the majority, they are hoping that they will be able to deny him outright by a vote. also, if they hold a majority, they can prevent the seat being filled by any other stauch conservative and force Trump to nominate, at minimum a moderate which would better serve their agenda.
i believe the court is considered to be an even split right now with Chief Justice Neil Gorsuch, Justice Clarence Thomas, Justice Samuel Alito, and Justice Anthony Kennedy all leaning right and Justice Elena Kagan, Justice Ruth Bader Ginsburg, Justice Stephen Breyer, and Justice Sonia Sotomayor all leaning left. so if Kavanaugh gets on, it will tilt the court to the right and it is widely suspected that RBG may not outlast Trump, so they are doubly scared.