Donald Trump has recently revived the birther debate by calling into question Sen. Ted Cruz's citizenship in the United States, citing his birth in the American Embassy in Canada as reason for suspicion. For Trump this is old ground, as he did almost the exact same thing last presidential election with President Barack Obama. By questioning Cruz's place of birth, Trump plays on his audience's pathos and instills fear of a nominee who might not have his constituents best interests in mind- if he's able to become president at all.
Trump plays on Republicans' fear of losing to the Democratic Party's candidate by introducing the idea that, once chosen as the Republican candidate, Cruz won't be able to be president at all. If Cruz is in fact not an American citizen (he undoubtably is an American citizen, but that doesn't seem to matter to Trump) he could conceivably be chosen as the Republican party's candidate and then be forced out of the election, handing it to the Democratic party's nominee. For those who would do anything to stop the Democrats from getting yet another term in office, this alone may be enough to turn them away from what could seem to be a "risky bet"
Trump also cashes on in the xenophobia exhibited by some of the American public. By painting Cruz as an outsider, Trump implies that Cruz is of the "other" and might in fact have ulterior motives that run against what's best for America. Trump provides his home-grown American self as a better option in comparison to the scary, ethnic, possibly Canadian Cruz.