President Obama on Tuesday issued his strongest warning yet about the Republican candidates for president with a two-word message on why voters should choose solemnly: nuclear codes.br Bluntly questioning front-runner Donald Trump's temperament, Obama said, "Whoever is standing where I'm standing right now has the nuclear codes with them and can order 21-year-olds into a firefight."br A restive electorate ultimately will decline to elect Trump, he predicted.br "The American people are pretty sensible," Obama said.br "And I think they'll make a sensible choice in the end."br The words represented Obama's most energetic criticism of the Republicans running to replace him.br For months, he has mostly kept a studied distance from the fray and resisted invitations to engage in political analysis.