  Phase shifts and inelasticity parameters for $NN$ scattering in the
partial-wave channels ${}^3S_1$--${}^3D_1$ and ${}^1S_0$ at energies $T_{\rm
lab}$ from zero to about 1 GeV are described within a unified $NN$ potential
model assuming the formation of isoscalar and isovector dibaryon resonances
near the $NN^*(1440)$ threshold. Evidence for these near-threshold resonances
is actually found in the recent WASA experiments on single- and double-pion
production in $NN$ collisions. There, the excitation of the Roper resonance
$N^*(1440)$ exhibits a structure in the energy dependence of the total cross
section, which corresponds to the formation of dibaryon states with
$I(J^\pi)=0(1^+)$ and $1(0^+)$ at the $NN^*(1440)$ threshold. These two
$S$-wave dibaryon resonances may provide a new insight into the nature of the
strong $NN$ interaction at low and intermediate energies.
