Kasich running i s not going to make Trump lose.
What exists right now are FIVE (lol) groups:
1) pro-Trumpers - about 15% of voters
2) pro-working class activists - about 30% of voters
3) anti-Trumpers - about 25%
4) the pro-Antifa far left - about 20%
5) Republican loyalists about 10% of voters
Groups 1 and 2 are voting Trump come hell or high water.
The third and fourth groups are not going to vote for Trump, but might split their vote.
The last category are the Republican loyalists many of whom sat out the last election. Once a sitting President has had four full years to work, he tends to pick up his own party loyalists.
All Kasich will wind up accomplishing is to split off Anti-Trump Republicans (Never Trumpers) and some more liberal Party loyalists who may end up feeling Trump is harming the party and the nation.
The way to beat Trump is for the Democrats to put forward a candidate who is liberal enough to get the Sanders people and moderate enough to let the Never Trumper Republicans and moderate Republicans to vote for them.
I highly doubt that the Democrats could find such a person even if they wanted to.
Most of them have been run out of the Democratic Party.