The nation will suffer if Trump secures the nomination of any major political party, even the Republican party. We have all seen that the most reliable way to cause the Republican party to hate any initiative is for Barack Obama to express his enthusiasm for it.
Therefore, for the good of the United States of America, the President should endorse Donald Trump. This will cause the Republican party to (finally) reject Trump's candidacy, resulting in an automatic improvement in the political discourse in the country.
Unfortunately, I do understand that this would require that the President, well, lie to the American people. Whilst this is regrettable, it's not a fatal flaw in this plan: a little fib is a small price to pay for the health of the Republic for which we all stand