Election 2016: What to Know About the Presidential Race Today

Ben Carson, who will hold a news conference where pastors are expected to endorse him, falls behind Donald J. Trump in a new national poll, a month after they were tied. The poll also shows Hillary Clinton staying ahead of Bernie Sanders just two months before the Iowa caucuses.

On the trail: Mr. Carson and Carly Fiorina are in South Carolina, Mrs. Clinton is in Florida, Mr. Trump is in Virginia and Jeb Bush is in Iowa.

Almost everyone in the top ranks of the Republican Party agrees that something has to be done about Mr. Trump. But few are willing to do it, even as irritation gives way to panic as it becomes increasingly plausible that he could be the party’s presidential nominee and that he could hurt other Republicans’ careers.

Many Muslim leaders in New Jersey feel a sense of betrayal as Chris Christie, who once rejected alarmist rhetoric about Islam, has called for Syrian refugees to be denied entry and has been restrained in his response to his rivals' remarks about Muslims. A New Jersey newspaper issued a sharp rebuttal to the endorsement of Mr. Christie by an influential New Hampshire newspaper.

Rudolph W. Giuliani says that he remembers some pockets of celebration in New York City on Sept. 11, 2001, but that Mr. Trump’s claim about thousands cheering in New Jersey was an exaggeration. To defend Mr. Trump, many supporters have pointed to a television report on reaction to the attacks. That, too, has been debunked.

As the Islamic State rivals the economy as a top concern in many polls, voters want a leader who makes them feel safe, like Harrison Ford in “Air Force One.” But some voters say the desire to be in control has led to some ugly moments.

In an event commemorating the day Rosa Parks refused to give up her bus seat in Montgomery, Ala., Mrs. Clinton called for changes in the criminal justice system and said there is something “profoundly wrong” when black men are disproportionately stopped and searched by the police, arrested or killed.