/* Electron attraction mediated by Coulomb repulsion */
According to my theory, this mechanism (despite its confusing labeling in Nature article) is actually responsible for all cases of superconductivity: the electrons become superconductive, when they're compressed each other. Which indeed requires attractive forces of another electrons, which are holding and squeezing them with outside structures (orbitals, crystal lattice or even polymer chains) like hens in the cages.
For example niobium gets superconductive, because it's held together with elongated f-orbitals, whereas the electrons in another orbitals remain squeezed inside of them. The sodium has no elongated orbitals available, therefore it remains soft and non-superconductive even at high pressure (which usually helps the superconductivity to manifest itself).
As the above article also notes, this mechanism has been proposed before fifty years already for synthesis of organic superconductors and these superconductors were really found in 1984 (i.e. some twenty years later) accidentally and named ultraconductors by team of Dr. Leonid Grigorov, in Moscow at the Institute of Polymer Materials of the Russian Academy of Science. They have also been reproduced at the Bar Ilan University in Israel where magnetic measurements showed stability at 9 Tesla, the limit of their equipment. Unfortunately that time the physics of superconductors has been dominated by BCS theory, so that the above proposal including the finding later has been ignored completely with mainstream physics (not quite surprisingly for regular visitors of cold fusion forums - the contemporary physics is full of such taboos).