A former Bush administration official said Wednesday that single-issue Republican voters, who plan to select a candidate based on their foreign policy, should consider likely Democratic presidential nominee Hillary Clinton if Donald Trump is her opponent come November.

"If you care about that bucket of social issues, pro-life, what not, you can't make the leap to Hillary Clinton," Nicolle Wallace, who served as President George W. Bush's communications chief and a senior adviser to the McCain-Palin campaign in 2008, told MSNBC in an interview Wednesday morning.

"But if foreign policy is how you vote, if that is your central concern, if counterterrorism is what worries you, how do you not consider Hillary Clinton in November?" she added.

Earlier in the program, Wallace said "the conversations happening in private with Republican consultants is that if you are not a social conservative, there is less and less rationale for hardened opposition to Hillary Clinton."

The campaign veteran's comments come just 24 hours after Trump responded to coordinated terrorist attacks in Brussels by suggesting U.S. leaders should "close our borders." He also vowed to reintroduce waterboarding and other enhanced interrogation techniques, if he's elected president.

"Brussels was a beautiful city, a beautiful place with zero crime, and now it's a disaster city," Trump told Fox News Tuesday morning. "It's a total disaster, and we have to be very careful in the United States, we have to be very careful and very vigilant as to who we allow in this country."

An ABC News/ Washington Post survey earlier this month found that a majority of Americans (61 percent) view Clinton as the most-qualified candidate to handle international crises, compared to 32 percent who said the same of Trump. The former secretary of state also led Trump by 14 percentage points on who voters trust more to combat terrorism.