Ted Cruz is strongly contesting Donald J. Trump nationally, according to a new Quinnipiac University poll that shows the Texas senator essentially deadlocked with real estate mogul and front-runner in the race for the Republican presidential nomination. According to the survey of registered Republican voters, 28 percent support Mr. Trump, while 24 percent back Mr. Cruz. The poll has a margin of error of plus or minus four percentage points. Trailing Mr. Trump and Mr. Cruz are Senator Marco Rubio and Ben Carson at 12 percent and 10 percent. The rest of the pack is languishing in the single digits....