  We study the elastic electric and magnetic form factors of the proton,
neutron and the charged roper resonance ($G_E^p$, $G_M^p$, $G_E^n$, $G_M^n$,
$G_E^R$ and $G_M^R$) systematically in a constituent quark model. Three
ingredients are crucial in this study: i) the mixing between the pure S-wave
and other components which produces a nonzero neutron electric form factor. ii)
a running coupling constant that soften the form factors. iii) the running
quark mass function, $M_q(p^2)$, which is responsible for the decreasing of the
$\mu_p G_E^p(Q^2)/G_M^p(Q^2)$ as $Q^2$ increases. The produced elastic form
factors of the proton and neutron match the corresponding observed values
fairly well. Our study shows that $\mu_p G_E^p(Q^2)/G_M^p(Q^2) \approx
M_q(Q^2/9)/M_q(0)$ upto $Q^2 \approx 4 \text{ GeV}^2$. We give predictions on
the elastic form factors of the roper resonance, the electric charge and the
magnetic momentum radius ratios of the roper resonance to the proton are
$r^R_{E}/r^p_{E} \approx r^R_{M}/r^p_{M} \approx 1.5$.
