Mitt Romney and Rick Santorum are in a virtual tie in Michigan, site of the Feb. 28 Republican presidential primary, according to a new NBC News/Marist poll.

Romney, the former Massachusetts governor who grew up in the Detroit area, had 37 percent. Santorum, the former U.S. senator from Pennsylvania, trailed with 35 percent. Texas Rep. Ron Paul had 13 percent, while former House Speaker Newt Gingrich had 8 percent.

The candidates debate tonight in Arizona, site of another Feb. 28 primary, starting at 8 P.M. EST. The NBC/Marist poll showed Romney with a wide lead in Arizona.

Michigan is considered a key battleground, since a home state loss would be an embarrassment for Romney.The poll found him benefitting from a perception he is the best GOP bet to beat President Barack Obama in the fall.

Marist found that 51 percent thought Romney had the best chance to win; only 24 percent thought Santorum had the best chance. And 52 percent of those polled said that characteristic was the most important factor in their vote.

The poll was conducted Feb. 19-20. 715 likely GOP voters were surveyed. Margin of error is plus or minus 3.7 percentage points.

Mitt Romney kicked off what's likely to be lively week of campaigning with a fresh blast at Rick Santorum. Romney is pushing hard to win the Feb. 28 Michigan presidential primary.

Polls show Santorum, the former U.S. senator from Pennsylvania, leading. Romney grew up in the Detroit area; his father was governor of Michigan from 1963 to 1969.

Romney and his supporters went negative in Florida and buried Newt Gingrich, the former House speaker. Monday, Romney's camp tore into Santorum.

“Mitt Romney spent his career in the private sector as a chief executive, where he helped create jobs and turn around major enterprises. Congressman/Senator Rick Santorum, on the other hand, has spent his career in Washington. Rick Santorum is part of the problem, so he can’t be part of the solution," said spokeswoman Andrea Saul.

Her comments were accompanied by a long statement about Santorum's time in Washington--two terms as a House member, two as a U.S. senator.

Romney was governor of Massachusetts from 2003 to 2007, and tried to win a U.S. Senate seat. He lost a bid for a Massachusetts seat in 1994 to Sen. Edward Kennedy.

Former New Hampshire Gov. John Sununu, a Mitt Romney supporter, blasted Rick Santorum Friday for backing earmarks, or local projects lawmakers insert into spending bills.

"Senator Santorum is a nice man. I’ve known him for a long time. He’s a good friend," Sununu told reporters in a conference call arranged by the Romney campaign.

"But frankly when he got to Washington, he got very Washingtonian," said Sununu of the former Pennsylvania senator.

"He fell in love with earmarks to the point where he is willing to say he was proud of being part of the earmark gang. And that increase in spending is a very serious problem," he said.

Santorum has said he backed earmarks because they helped the state he represented. He and Romney, the former governor of Massachusetts, are battling to win Michigan's Feb. 28 presidential primary.

Ohio Attorney General Mike DeWine switched his support Friday from Mitt Romney to Rick Santorum.

"To be elected President, you have to do more than tear down your opponents," DeWine said in an announcement on the steps of the Ohio Statehouse in Columbus.

"You have to give the American people a reason to vote for you -- a reason to hope -- a reason to believe that under your leadership, America will be better.  Rick Santorum has done that.  Sadly, Governor Romney has not."

DeWine served with Santorum, a former Pennsylvania senator, in the U.S.  Senate

Romney and his supporters bombared Florida with negative ads, hoping to defeat former House Speaker Newt Gingrich. The Romney campaign lately has been criticizing Santorum for his spending record. Santorum leads Romney in polls in Michigan, site of a crucial Feb. 28 presidential primary. Ohio voters go to the polls a week later.

Foster Friess, who startled Andrea Mitchell and sparked a Twitter-storm Thursday with his remarks about birth control and aspirin, has apologized for the comments, saying he was just making a joke -- but that it fell flat.

In a blog posting on Foster's Campfire Blog, Friess, who is bankrolling a pro-Santorum super PAC, said his "aspirin joke bombed as many didn't recognize it as a joke but thought it was my prescription for today's birth control practices. In fact, the only positive comments I got were from folks who remembered it from 50 years back. Birth control pills weren’t yet available, so everyone laughed at the silliness on how an aspirin could become a birth control pill.

"After listening to the segment tonight, I can understand how I confused people with the way I worded the joke and their taking offense is very understandable. To all those who took my joke as modern day approach I deeply apologize and seek your forgiveness. My wife constantly tells me I need new material—she understood the joke but didn’t like it anyway—so I will keep that old one in the past where it belongs."

His apology comes as Santorum sought to distance himself from what he called a "bad joke...It's not reflective of me or my record on this issue," he said on CBS's This Morning. Santorum has said he's personally opposed to contraception, a spokesman told MSNBC, however, that he believes it's not up to the government to determine whether women can have access to them.

A major Rick Santorum supporter today raised eyebrows today by suggesting women could use aspirin as a contraceptive.

Foster Friess, a Wyoming multi-millionaire who is bankrolling a super PAC to run ads for Santorum, told MSNBC's Andrea Mitchell -- with a smile on his face -- that "back in my days, they used Bayer aspirin for contraceptives. The gals put it between their knees and it wasn't that costly."

Mitchell was at a loss for words and suggested changing the subject. She had asked Friess whether he agreed with Santorum's stance on social issues, including women in combat and contraceptives -- he is personally opposed to them. Friess suggested there are weightier issues than contraceptives that should be dealt with.

"Here we have millions of our fellow Americans unemployed," he said. "We have jihadist camps being set up in central – in Latin America, which Rick has been warning about, and people seem to be so preoccupied with sex. I think it says something about our culture. We maybe need a massive therapy session so we can concentrate on what the real issues are."

An hour before former Pennsylvania Sen. Rick Santorum was to speak to the Conservative Political Action Conference in Washington, Mitt Romney released a blast at his rival.

"tSenator Santorum now decries the obscene growth in federal spending and our national debt, but he was in Washington as this fiscal crisis grew – supporting billions in earmarks, repeatedly voting to raise the debt ceiling, and admitting that deficits no longer bothered him," Romney spokesman Andrea Saul said in a memo to media.

"Americans want a president who can offer solutions, not a lifelong politician who was part of the problem in the first place."

The Romney camp then recalls statements Santorum has made supporting earmarks, or local projects lawmakers routinely insert into spending legislation. Santorum has explained that was part of his job.

Santorum is popular among conservatives; Romney, less so. Both are to speak to the conference Friday.

Mitt Romney could be in for a rough night Tuesday, as polls suggest former Pennsylvania Sen. Rick Santorum has a chance to win the Minnesota caucuses.

Romney's camp put out a memo Tuesday morning that looks ahead, and perhaps tries to lessen the impact of the Tuesday result.

"It is difficult to see what (former Massachusetts) Governor Romney’s opponents can do to change the dynamics of the race in February," said political director Rich Beeson in the memo. "No delegates will be awarded on February 7 -- Colorado and Minnesota hold caucuses with nonbinding preference polls, and the Missouri primary is purely a beauty contest.

"Except for the Maine and Wyoming nonbinding caucuses running through February, the next contests are on February 28 in states where Governor Romney is strong. Arizona’s 29 delegates will be bound in a winner-take-all contest. Michigan, the state where Governor Romney grew up, binds 30 delegates"

Beeson then looked ahead to March, with optimism.

"The rules for the March states offer even less comfort to Governor Romney’s opponents. With so many states and territories voting, organization and resources are key. Ours is the only campaign to be active in all of these states, and we have the resources and organization to maximize delegate totals according to each state’s rules," he said.

Santorum, former House Speaker Newt Gingrich and Texas Rep. Ron Paul all see possibilities in March. Gingrich and Santorum eye southern states holding primaries and caucuses, where GOP voters tend to be more conservative. Paul is pushing hard in caucus states.

Beeson says he's unworried. "Governor Romney is the only candidate prepared to compete in simultaneous contests across the country," he said.

"Speaker Gingrich’s and Senator Santorum’s campaigns have resource challenges. The remaining February states may not be kind to them, and their hopes for a comeback in March may be very difficult and based on an incomplete understanding of the delegate selection rules."

Just in time for the rest of the caucuses and primaries, trading card manufacturer Upper Deck is releasing "World of Politics" trading cards featuring the presidential contenders: from still-in-the-running President Barack Obama, Mitt Romney, Newt Gingrich, Ron Paul and Rick Santorum to also-rans like Rick Perry and Herman Cain.

The California-based company says nine subjects will be featured in all: "With so much attention around the 2012 election, we thought it was only fitting to produce a set of cards that would help pay tribute to all the highs and lows of this presidential race by capturing the top candidates on cardboard," said Jason Masherah, Upper Deck's marketing veep. The cards will be released Feb. 21.

It's not Obama's first time on a trading card: the trading card company released a caricature of Obama after he won in 2008. 

Younger voters have tended to be Mitt Romney's weakest age group, but he won more support Tuesday from voters under 30 than any of the Republican candidates.

The Center for Information & Research on Civic Learning & Engagement, which studies youth voting issues, found in a preliminary analysis that Romney topped Texas Rep. Ron Paul by 16 percentage points.

About 100,000 young Floridians voted in the Tuesday GOP primary. Romney got 41 percent. Paul had 25 percent, followed by former House Speaker Newt Gingrich, 21 percent, and former Pennsylvania Sen. Rick Santorum, 13 percent.