If it is Phlox paniculata, you could take cuttings from the new emergent growth in spring. The shoots they have now will probably not root though.

You can take root cuttings in spring too. Lift the plant just before coming into growth and select a couple of the thickest roots. Cut them into 2 inch sections and lay horizontally in seed tray with seed compost. Just cover them and place in light shade and keep slightly moist.

The other method is to divide the root stock in early spring before growth.

I seem to recall that if the parent plants have a virus, root cuttings from the plant are always free of it.