  The globular cluster NGC 2419 was the first to exhibit a Mg-K
anticorrelation, linked to hydrogen burning at temperatures between 80-260 MK.
However, the key K-destroying reaction,
$^{39}\mathrm{K}(p,\gamma)^{40}\mathrm{Ca}$, has a large rate uncertainty in
this range. We significantly constrain this rate with a high resolution
$^{39}\mathrm{K}(^{3}\mathrm{He},d)^{40}\mathrm{Ca}$ study. We resolve the
E$_{\text{r}}^{\text{c.m.}} = 154$ keV resonance in $^{39}\mathrm{K}+p$ for the
first time, increasing the previous rate by up to a factor 13 and reducing its
$1\sigma$ width by up to a factor of 42. Reaction network calculations for NGC
2419 suggest that this could lower temperatures needed to reproduce the Mg-K
anticorrelation.
