Republicans, like other American citizens, have the right and the obligation to vote their conscience. If after due consideration, individual voters choose Trump, the our system of democracy has worked the right way again. There's also no need for the party leadership to support only one Republican candidate for President.
If the people the Republicans represent support Donald Trump then they are obligated to support him as well. If their people do not support Donald Trump then they should not support him. As polarizing as he is, I believe politicians should set their own views aside and do what their people want.
No, the Republicans do not have an obligation to support Trump as a nominee. Trump is the best known of the Republican candidates and he seems to be the front runner for the party. He has received the majority of the press for the Republicans for this presidential election. This does not mean that Republicans are not free to support someone else.
The Republicans should be able to withdraw support of any nominee at any time. Supporting an inadequate candidate is pointless, as an inadequate candidate will not win. If the Republicans decide that Trump should no longer have their support, they should be able to withdraw that support. Support of a nominee should not be based solely on which political party supports that nominee.