Kansas, Kentucky, Louisiana and Maine are holding Republican contests today. If the establishment effort to stop Trump has any chance of working, they will need to see Trump under-perform today. Trump has an average of a 16 point lead over Cruz in Louisiana, a 13 point lead over Rubio in the only Kentucky poll we have, and we don't have Maine polling. Kansas polling is minimal, although what we have puts Trump ahead of Cruz. You can check them at RealClearPolitics. Betting odds currently favor Cruz in Kansas, though. It is a caucus with a lot of evangelicals. In other words, it looks like Iowa, where Cruz beat Trump. Polling for a caucus is difficult because turnout is harder to predict.
So, if Trump loses Kansas, that changes nothing. In order for the "Never Trump" movement to have a chance, they need to cut deeply into Trump's leads in Kentucky and Louisiana.
A surprise victory by a non-Trump in either of those states would put this race back in play.