To have one’s name put in nomination, he has to have the majority of delegates in 8 states. By March 15, 26 states plus Puerto Rico will have voted. Who besides Trump, at this point can make that requirement? Somebody may have one or two, but Trump will be the only one over the 8 delegation threshold as it stands now.
It’s why the donor class forced Walker out (probably threatening to not fund his Wisconsin reelection if he lost the prez bid). They know the only way they can win is to force a lot of RINOs out and try to consolidate the vote around one of them. They’re apoplectic over this, especially since they don’t quite know which one to go with. They’re basically going to Rubio by default because he polls the least negative of all of their side.
If Trump remains in the 25-30% neighborhood, that could be hard to do.