Yes, a few Muslim citizens have engaged in terrorism against the US, but an overwhelming majority of Muslims are productive and loyal citizens. It is no secret that Muslim bashing is popular among some in the Republican Party. They conveniently forget that some of the plots were thwarted by Muslims contacting the police.
Sadly not one candidate on the podium admonished Cain and Gingrich, as they would have, if such a sweeping statement was made to malign other minorities such as Jews or Blacks. Even sadder yet, it did not elicit much comment in the popular media either.
Not too long ago some Republicans did stand up to challenge such remarks, but the party has changed.
In 2007 Republican debates, while others remained quiet, John McCain condemned Mitt Romney's remark that he will not appoint any Muslim to his cabinet. McCain said , "I'm proud of the Muslims who are currently serving in the United States armed forces and my sense is that if they can serve in that manner, they can serve in any position of responsibility in America."
In December 2002, an intemperate remark by Senator Trent Lott cost him the leadership of the Republican Party in the Senate. Celebrating the 100th birthday of South Carolina Senator Strom Thurmond, Senator Trent Lott, the Republican leader in the Senate said, "When Strom Thurmond ran for president, we voted for him. We're proud of it. And if the rest of the country had followed our lead, we wouldn't have had all these problems over the years, either."
Those remarks were construed as racist, because in 1948 Strom Thurmond broke with the Democratic Party on the issue of enforcement of civil rights legislation and ran for the Presidency as a State's Rights Party candidate.
The US Constitution is secular. Thankfully our laws are not based on religious preference. Although when not in conflict with the civil laws, in some jurisdictions it has accommodated people, who by mutual agreement wanted their personal disputes settled by arbitration by religious tribunal. Courts have enforced the result of such arbitration in case of Jewish, Mormon and other religions as in contract law.
The spurious injection of Sharia in the debate is a "Red Herring" to divert attention and garner cheap popularity from the considerable weight of the xenophobic wing of the Republican Party. There is no monolithic Sharia law. These laws were developed by different schools of thoughts in the 9th century to check the creeping autocracy of the rulers. They are open to debate, and have evolved on diverse lines through the ages.