Republican presidential candidate and ex-Massachusetts governor Mitt Romney pauses during a speech to supporters Tuesday night in Boston. REUTERS

The failure by Mitt Romney to secure a decisive victory on Super Tuesday underscores nagging questions about his candidacy and seems to ensure a protracted battle for the Republican presidential nomination.

Romney sailed to easy victories in Vermont, Massachusetts, Virginia, Idaho and Alaska but was unable to reproduce those results in the South or in the battleground state and general-election bellwether of Ohio. Exit polls showed that Romney continued to struggle to attract lower-income voters as well as conservatives.

The states voting Tuesday award delegates proportionately, so Romney will retain his lead in the delegate count -- a point his campaign is likely to stress in arguing that the former Massachusetts governor should get the nomination. But his inability to take Tennessee or win by a big margin in Ohio despite superior financing and a parade of endorsements from party leaders dealt a blow to Romney's contention that he is the inevitable nominee.

The night's biggest prize was Ohio, from which every Republican primary winner since 1972 has gone on to win the presidential nod. The state will be fiercely contested in the general election, but Romney beat Rick Santorum by only one percentage point.

Exit polls exhibited a familiar pattern: Romney defeated Santorum among voters with a college degree and who make more than $100,000 a year, while Santorum attracted less-educated voters and those earning under $100,000.

Self-avowed very conservative voters chose Santorum by a 2-to-1 ratio, while the ideological spectrum to the left of that bloc gravitated toward Romney. In terms of the quality that voters most sought in a candidate, Romney won among those looking for one who can defeat President Obama. Santorum was a hit among those more concerned with principle than pragmatism: Is a true conservative and has strong moral character went for the former Pennsylvania senator.

Santorum, who has emerged as Romney's chief rival by positioning himself as the real conservative in the race, also scored Super Tuesday primary victories in Oklahoma and Tennessee (as well as in North Dakota's nonbinding caucus). Both Southern states have large populations of conservative and evangelical Christian voters, and Santorum's unequivocal wins in both will likely reignite questions about Romney's chances among skeptical, bedrock conservatives.

I've never been for an individual mandate on a state or federal level, Santorum said in a speech, alluding to Romney's support for a health care overhaul in Massachusetts that became a model for Obama's national legislation. We need a person running against President Obama who is right on the issues and truthful with the American public, Santorum added.

Newt Gingrich handily won Georgia, the state he represented for years in Congress. Gingrich has seen his campaign falter since crushing Romney in the South Carolina primary in January, and he invested significant time and resources in trying to revive his campaign with a Georgia win. The Gingrich campaign has said the ex-House speaker's road to the nomination runs through the South, and while he failed to win in Oklahoma and Tennessee, Gingrich vowed to press on.

I want you to know that in the morning we're going on to Alabama, we're going on to Mississippi, we're going on to Kansas and that's just this week, he told supporters Tuesday night in Atlanta.