  Semilocal and electroweak strings are well-known to be unstable against
unwinding by the condensation of the second Higgs component in their cores. A
large class of current models of dark matter contains dark scalar fields
coupled to the Higgs sector of the Standard Model (Higgs portal) and/or dark
U(1) gauge fields. It is shown, that Higgs-portal-type couplings and a gauge
kinetic mixing term of the dark U(1) gauge field have a significant stabilising
effect on semilocal strings in the "visible" sector.
