Rick Santorum has confirmed that if elected as president of the US, he would support a constitutional amendment invalidating existing gay marriages.

In an interview with NBC at his campaign headquarters for the Iowa caucus, he said there should be one marriage law across all states, and that he would support a federal amendment to the constitution which had the effect of dissolving existing unions.

Answering a question on whether gay couples would be forced to divorce under the amendment, he said “Well, their marriage would be invalid. If the constitution says ‘Marriage is this’, then people whose marriages are not consistent with the constitution…”

He shrugs as the interviewer then asks if that would be the only solution, to which he says he wishes there were “another way”.

Santorum had had a last minute surge to third place in a CNN/Time/ORC opinion poll ahead of the first day in the Republican presidential candidate selection process.

He said “special” privileges were necessary to allow men and women to marry and raise children for the benefit of society.

After adding that “traditional” marriage had been voted for 32 times across 32 states, he tackled the growing public support for gay marriage rights, saying some people used to think “blacks were less than human […] Well, just because public opinion says something, doesn’t mean something is right if it is not right”.

He then says he bases his decision on “reason and faith”, adding: “If your reason is right and your faith is true, you are going to end up in the same place”.

Santorum has been vocal in his opposition to equal marriage rights. In July 2011 he said the political left were in favour of gay marriage because it was a “two-fer”.

He explained: “When you redefine marriage, you cheapen marriage. You make it into something less valuable, less special … [and] it is a sure bet that will undermine faith.”

The Iowa caucuses will take place at 7pm local time today, where registered Republican voters make their choice for the party’s presidential candidate.

A similar process is repeated throughout the other states. The Democrats will hold caucuses but will not choose a candidate as President Obama is running for re-election.

While the results of the Iowa caucuses will usually indicate Iowa’s choice of candidate, they do not consistently predict the final candidate, but are seen to narrow the field ahead of a final decision and the presidential election in November.

In the final poll before voting opened, Mitt Romney and Ron Paul appeared tied for first place, with Newt Gingrich in third.