DETROIT -- In a sign of frustration that he is still competing with former Pennsylvania Sen. Rick Santorum for the Republican presidential nomination, former Massachusetts Gov. Mitt Romney on Tuesday called his opponent a "lightweight."

Romney visited his campaign headquarters in the suburb of Livonia as voters went to the polls to pick the Republican nominee, and tore into Santorum for having his campaign send robocalls to Michigan Democrats that hit Romney for opposing the auto bailouts and asked for their votes.

But he also jabbed at Santorum's private sector credentials, trying to turn the conversation back to his central theme.

"I think he's an economic lightweight," Romney said. "He doesn't have the very attribute and skill that is most on demand on the part of the American people right now."

It's the latest salvo in a back and forth between Romney and Santorum that has come to resemble, if not quite match, the furious exchanges between Romney and former House Speaker Newt Gingrich leading up to Florida's Jan. 31 primary, which Romney won.

And it was the first time that Romney took questions from the press in almost three weeks, according to ABC News' Emily Friedman.

Romney has attacked Santorum most aggressively for his robocalls, which were first reported Monday afternoon by Talking Points Memo.

On Fox News early Tuesday morning, Romney referred to the calls as "outrageous and disgusting, a terrible dirty trick at the last hour," and "a new low for his campaign."

Santorum had previously pointed out that Romney's campaign was doing robocalls in which it played a recording of Santorum endorsing Romney in 2008.

"Calling Democrats who are eligible to vote here to vote for us, that's a low blow? Encouraging people to come and vote for us and we talk about our manufacturing plant and what we're going to do to create jobs?" Santorum told Fox's Sean Hannity Monday night. "It's a very positive robo call.”

But Romney faced questions Tuesday morning about why he voted for former Sen. Paul Tsongas (D-Mass.) in the 1992 Democratic presidential primary.

"When there was no real contest in the Republican primary, I’d vote in the Democrat primary, vote for the person who I thought would be the weakest opponent for the Republican," Romney told ABC's George Stephanopoulos in an interview running up to the 2008 election.

When asked about this vote Tuesday morning at a press conference with reporters, Romney said it was "very different being a candidate for president and buying ads," as Santorum has done, and complained that Santorum does not mention his own opposition to the auto bailouts in the ads.

And Romney also acknowledged, again, that he was trying to undermine the Democrats, as Michigan Democrats are now trying to do to him.

"I wanted someone other than Bill Clinton. I voted against Ted Kennedy, Tip O'Neill and Bill Clinton. Seemed like a good group to be against," Romney said.

He has accused Santorum of working with Democrats to undermine the election result, while the Santorum campaign has responded that it is simply trying to attract support from blue-collar conservative Democrats.

Romney was also asked about Santorum's comments on the separation of church and state, particularly that former President John F. Kennedy's speech on the topic made him want to "throw up."

Romney took a variation of Santorum's position on the issue, while subtly suggesting that his rhetoric was over the top.

The idea of separation of church and state, Romney said, "has been used by some to demand that there is no recognition of God in society."

"I do not believe we should establish secularism as a new religion in America," he said. "At the same time I respect President Kennedy and his expression of his own views."

CORRECTION: An earlier version of this article incorrectly stated the date of Mitt Romney's interview with ABC News.

Also on HuffPost: