Republican presidential hopeful Mitt Romney received backing from major Republican figures on Wednesday after a big win in Illinois, but an aide's gaffe reinforced qualms about his campaign.

Former Florida governor Jeb Bush -- whose father and brother are both ex-presidents -- threw his considerable weight behind Romney after the frontrunner romped to victory in Tuesday's Illinois primary.

"Now is the time for Republicans to unite behind governor Romney and take our message of fiscal conservatism and job creation to all voters this fall," Bush said in a statement.

"We face huge challenges, and we need a leader who understands the economy (and) recognizes more government regulation is not the answer," he said.

But a senior advisor's comparison of the former Massachusetts governor's strategy to an "Etch A Sketch" toy reinforced core Republican doubts about Romney's conservative credentials and overshadowed Bush's endorsement.

Asked on CNN whether the primary had pushed Romney too far to the right for general election voters, advisor Eric Fehrnstrom said "I think you hit a reset button for the fall campaign. Everything changes."

"It's almost like an Etch A Sketch. You can kind of shake it up and restart all over again."

Senator John McCain meanwhile said Romney could become the Republican party's de facto nominee long before he wins the 1,144 delegates needed to clinch the nomination to face President Barack Obama in November.

"I don't know if they will 'drop out' or not," McCain said of Romney's three remaining rivals, in an interview with AFP.

"But at some point, and I think we may have reached that point in Illinois, the inevitability factor comes into play, so that whether they are still in theoretically or not, that they're no longer hotly contested states."

Conservative underdog Rick Santorum, unbowed by Romney's widening lead after Illinois, vowed to take the fight all the way to the Republican convention in August and was quick to seize on the gaffe at a campaign stop in Louisiana.

"The reason governor Romney hasn't been able to seal the deal" is because "a lot of voters" know "he will say what he needs to say to win the election."

"We've had enough of politicians like that in this country," Santorum said.

As the mocking from Santorum, fellow candidate Newt Gingrich and Democrats piled up, Romney went into damage control mode.

"Organizationally, a general election campaign takes on a different profile," he told reporters, but "the issues I'm running on will be exactly the same."

"I'm running as a conservative Republican, I was a conservative Republican governor, I'll be running as a conservative Republican nominee for president."

With many states awarding delegates proportionately and with his rival refusing to bow out, analysts say it could take Romney until June to win sufficient delegates to clinch the nomination.

A bloody floor fight, while unlikely, is also still possible if Santorum succeeds in piercing the former Massachusetts governor's air of inevitability.

Romney meanwhile portrayed himself as the virtual nominee at a campaign stop in Maryland, which holds its primary on April 3.

Not once did he mention his Republican rivals, instead training his sights on the economic failings of the man in the White House.

"I see our president attacking economic freedom, and he doesn't know what he's doing," he told hundreds of supporters at an American Legion hall in the town of Arbutus, as he drilled into Obama's failed policies and explained why Romney's economic chops would bring about a fuller recovery.

"When the president interferes with the free marketplace, he substitutes the heavy hand of government for the invisible hand of the market, to the peril of economic freedom. That must end."

Romney has now won 21 out of 33 nomination contests, while former Pennsylvania senator Santorum has notched up 10 wins -- largely with the help of evangelicals and the party's most conservative members.

Santorum could regain some momentum with a win on Saturday in the southern state of Louisiana, where a recent poll showed him with a 13-point lead.