The Republican presidential race is entering a crucial phase, but the latest polls show there is no clear front-runner to unseat Democratic U.S. President Barack Obama in November.

Former Pennsylvania senator Rick Santorum and former Massachusetts governor Mitt Romney have emerged as the two favorites among Republican voters. And a nationwide poll released Wednesday by Quinnipiac University shows Santorum surging ahead, leading Romney by 9 percentage points.

But polls find Santorum would lose to President Obama in the general election, while an Obama-Romney race would be too close to call.

Ahead of Wednesday night's nationally televised debate in Arizona, Romney went on the attack, saying Santorum's voting record in Congress is not that of a fiscal conservative.

Romney's comments come as his campaign is paying millions of dollars for ads trying to depict Santorum as a big spender.

Santorum says that is not true, saying he took on the tough issues, including Medicare, Medicaid and Social Security.

Political analysts say Wednesday night's debate – which will also feature former House Speaker Newt Gingrich and Texas Representative Ron Paul – could leave a strong impression on voters in Arizona and Michigan, which hold primaries next week.

On March 6, voters in 10 states head to the polls to choose a challenger to take on President Obama in November's general election. The day is known as “Super Tuesday.”