  Hole-doped cuprate superconductors show a ubiquitous tendency towards charge
order. Although onset of superconductivity is known to suppress charge order,
there has not so far been a decisive demonstration of the reverse process,
namely, the effect of charge order on superconductivity. To gain such
information, we report here the dependence of the critical temperature
$T_{\mathrm{c}}$ of YBa$_2$Cu$_3$O$_{6.67}$ on in-plane uniaxial stress up to 2
GPa. At a compression of about 1 GPa along the $a$ axis, 3D-correlated charge
density wave (3D CDW) order appears. We find that $T_{\mathrm{c}}$ decreases
steeply as the applied stress crosses 1 GPa, showing that the appearance of 3D
CDW order strongly suppresses superconductivity. Through the elastocaloric
effect we resolve the heat capacity anomaly at $T_{\mathrm{c}}$, and find that
it does not change drastically as the 3D CDW onsets, which shows that the
condensation energy of the 3D CDW is considerably less than that of the
superconductivity.
