WASHINGTON -- In his State of the Union address to Congress on Tuesday night, President Barack Obama never mentioned Donald Trump. But the president’s speech, stressing optimism, tolerance and good will, was from beginning to end a dismissal and rebuttal to everything the billionaire real estate bigot is and says. Trump, no one should ever forget, has called millions of immigrants from Mexico drug dealers and rapists, and has proposed an immediate and indefinite ban on immigration to this country of all Muslims seeking to enter from other countries. He has said that the “the American dream is dead;” that Obama’s actions -- or lack of action -- created ISIS; that the entire decision-making process of Washington is corrupt and useless; that the American economy (the strongest in the world) is a hopeless shambles; and that only he -- by virtue of his alleged brilliance and leadership skills -- can, singlehandedly, bring America back to life. Oh, and he vows to build a wall from Southern California to East Texas, and kick 11 million undocumented workers out of the country, and create a special force to handle the evictions, and to let people back in one by one, presumably on his say-so, by rules he would write himself. The depth of accumulated unreality and even insanity represented by these stands have propelled Trump into the lead in the race for the Republican presidential nomination. Two other GOP candidates are rising to challenge him, primarily by parroting Trump and Trumpism to the extent that they can. Indeed, Sen. Ted Cruz of Texas and Sen. Marco Rubio of Florida appear to be, at least at this moment, the only two challengers capable of stopping Trump -- and they are, in many ways, as hair-raising as he is. From time to time, America has given in to the darker implications of its historic, unique and vital role as a “nation of immigrants.” Each succeeding wave of immigration for the last 200 years -- Germans, Irish, Chinese, Italians, Slavs, Jews, Greeks, Latin Americans, South Asians and now Muslim Arabs -- has engendered fear among those already here and already established. There is no more fierce protector of American culture than a generation of immigrants that has just adopted this country as home. In wartime, especially, that dark side of the “nation of immigrants” tradition has led to nightmarish consequences: violence at times against German-Americans in World War 1; the internment of Japanese-Americans in World War 2. Religious prejudice has played a major role as well. A large portion of American politics in the 19th century was based on a fear of Catholics and the idea that the pope would have more temporal power here than the president. But never before has a prominent candidate for president -- indeed several prominent candidates, all the same party -- made xenophobia, racial and religious antagonism, and fear the organizing principles of their entire effort. Will the Republican Party pledge allegiance to this message and to one of these messengers, Donald Trump? If they do so, bringing their nominee within striking distance of winning the White House, what signal will other countries with more tribal histories -– a Germany, a France, a Turkey, a Sweden -- take from America’s failure to live up to its ideals? That is the big question framing the 2016 election, not only for the U.S. but for the world. We know that the world is watching: social media statistics for 2015 show that the U.S. election was the most followed news story on the planet -- and it wasn’t even officially the election year yet. The question becomes more urgent the closer we get to voting, which begins on Feb. 1 in GOP and Democratic caucuses in Iowa. In his State of the Union Address, Obama challenged Trump’s pessimism and his reliance on the fear of outsiders. Optimism and diversity, the president said, were not only American ideals, but American strengths, economically and socially. The world looks to American economic strength, Obama said, but also to our example of adaptability and acceptance of others. The U.S. leads diplomatically, he said, in party because of our willingness to deal equally with partners. "I reject any politics -- any politics -- that targets people because of race or religion,” the president said to wide applause. He added that this was “not a matter of political correctness,” but based on an understanding that the U.S. as a society and an economy thrives and is durable because of “diversity and openness.” To reject those principles, he said “betrays who we are as a country.” Obama framed the issue, and the question: "How do we remain open and diverse and yet safeguard the American people?” That is the central issue in the 2016 campaign, and the world is waiting to see what we do. _ALSO ON HUFFPOST:_ -- This feed and its contents are the property of The Huffington Post, and use is subject to our terms. It may be used for personal consumption, but may not be distributed on a website.