LIVONIA, MI -- As Michigan primary voters head to the polls in this pivotal, deadlocked contest, Mitt Romney today admitted he has made mistakes as he's campaigned in his birth state.

And he leveled new attacks meant to halt any late momentum for his rival Rick Santorum, calling out the former Pennsylvania senator for engaging in "dirty tricks" -- and also labeling him a "lightweight" on the economy.

"I'm very pleased with the campaign, its organization," Romney said this morning at his first press conference in nearly three weeks. "The candidate sometimes makes some mistakes, and so I'm trying to do better and work harder and make sure that we get our message across."

Romney refused to elaborate on his mistakes, telling the press he "can't imagine" they would have a hard time coming up with anything. He later acknowledged with a one-word "Yes" that comments this week that have highlighted his wealth -- including saying his wife owns two Cadillacs, and that he is friends with NASCAR team owners -- have been problematic for his campaign.

"Never repeat your mistakes," Romney told reporters gathered in his campaign headquarters here, before returning to his main focus today: Santorum.

Romney launched a new line of attack on Santorum, describing him as an "economic lightweight," unschooled in how to create jobs, or how the economy works. Romney even used the words of another candidate against the former Pennsylvania senator, who's campaign admitted yesterday it is reaching out to Democratic voters across Michigan, urging them to vote for Santorum in the GOP primary.

"I understand yesterday Newt Gingrich made a comment, which turns out to be pretty accurate. He said that Senator Santorum is a big labor Republican, and he has proved that again by doing the work of the UAW [United Auto Workers] and the Obama team," Romney said. "Linking up with them, taking one not for our team but for their team, and trying to get them to vote against me so that they can have someone who they think is easier to beat in the fall."

Santorum's effort to turn out Democrats against Romney was clearly a focus of his appearance this morning. Romney dispensed with the usual made-for-TV hallmark of his primary-day events: He never picked up a phone to call undecided voters. Instead, upon arriving with Michigan Gov. Rick Snyder in tow, he joked with his own volunteers.

"You are making calls to Republicans today. This is a good thing, all right," Romney said drily. "The Santorum campaign is making calls to Democrats today."

Romney told the assembled press, and a senior adviser later acknowledged as well, that the most difficult element about gauging which way this state will break, and whether it will be a photo finish or a clear win for either candidate, was the potential Democratic interference in the primary.

"I think Republicans have to recognize there’s a real effort to kidnap our primary process," Romney said. "And if we want Republicans to nominate the Republican who takes on Barack Obama, I need Republicans to get out and vote and say no to the ... dirty tricks of a desperate campaign." 

Romney also addressed the concerns raised by some about his apparent inability to excite the base of his party, saying he is not willing to "say anything to get the nod," and that he would continue his laser-like focus on the economy going forward.

"You know it’s very easy to excite the base with incendiary comments. We've seen throughout the campaign that if you're willing to say really outrageous things that are accusative and attacking of President Obama that you're going to jump up in the polls," Romney said. "You know I'm not willing to light my hair on fire to try and get support. I am who I am. I'm a person with extensive experience in the private sector and the economy."