Mitt Romney's campaign has said he is almost certain to be the US Republican presidential candidate, as his Democratic opponents mock his long "death march" to secure the nomination.

Despite collecting six victories in the 10-state bonanza known as Super Tuesday, including in crucial Ohio, the flawed favorite failed to knock out chief rival Rick Santorum and the race was set to drag on for weeks or months.

Romney's campaign has sought to keep up the air of inevitability he has maintained on and off for months, saying nothing short of "an act of God" would keep him from facing off against Democratic President Barack Obama in November.

But in the meantime, Democrats are savoring the prospect of more bitter Republican in-fighting and relishing the chance to fill Obama's campaign coffers as Romney drains his in the increasingly costly state-by-state slog.

Romney must now fend off Santorum and former House speaker Newt Gingrich in a part of the country where the former moderate governor of liberal Massachusetts has been the weakest -- the conservative Deep South -- with contests looming in Alabama, Mississippi, Louisiana and Missouri.

"All we have to do is keep doing what we're doing, and we can get the nomination," a senior Romney campaign official said Wednesday.

But as the Republican primary season has lingered, Obama's reelection chances have improved, with the economy sputtering back to life and his approval ratings inching back up to 50 percent.

"What was extraordinary about yesterday was that having outspent Santorum six to one and Gingrich four to one, Romney found himself at midnight, still wondering whether he was going to carry the state of Ohio, which he barely did," Obama's chief political strategist David Axelrod said.

"And so he continues to grind out... tactical victories in a kind of death march," he told a conference call with reporters.

Romney has fended off surges from several rivals in recent months, winning when it counted most in states like Florida and Ohio, even as he has struggled to connect with core conservatives suspicious of his centrist past.

The Romney campaign official insisted his lead and diminishing opportunities on the calendar for major delegate hauls make a "clear case that the nomination is an impossibility for Rick Santorum or Newt Gingrich."

"For those guys, it's going to take some sort of act of God to get to where they need to be on the nomination."

Yet they remain in the race, denying Romney the opportunity to put a negative Republican campaign behind him while at the same time splitting the conservative majority vote between them.

"I think the long slog continues for Mitt Romney towards the nomination," Dante Scala, a politics professor at the University of New Hampshire, told AFP.

Still, Super Tuesday left little doubt in analysts' minds that Romney will eventually secure the crown.

The multimillionaire former businessman can boast that he is a survivor, having come from behind to win big states like Florida and Ohio, two must-win battlegrounds for any Republican in the general election.

Romney's Ohio win, narrow as it was, also denied Santorum his central argument that he alone can compete against Obama in working-class, Midwestern swing states.

Romney, 64, has now won 14 states, Santorum seven and Gingrich two. But the next states in play could rob Romney of any forward movement.

Santorum, 53, a devout Roman Catholic who fiercely opposes abortion and gay marriage, was back on the campaign trail stumping in conservative Kansas on Wednesday ahead of two rallies in Mississippi.

Kansas votes on Saturday and the delegate-rich southern states of Alabama and Mississippi hold primaries Tuesday, all of which could prove a challenge to Romney and hand fresh victories to either Santorum or Gingrich.

Each state is allocated a certain number of delegates to the Republican Party's August convention, which will crown the nominee. To win, one of the candidates will need to secure 1,144 delegates by then.

Opportunities for decisive delegate hauls have dwindled down to just two dates.

Five states vote on April 24, including New York and Santorum's native Pennsylvania, with a total of 231 delegates at stake, while June 5 offers 299 delegates, including from voter-rich California.

The authoritative website Real Clear Politics put Romney's current delegate count at 404, Santorum at 161, Gingrich at 105 and Texas congressman Ron Paul at 61.