WASHINGTON (AP) — Mitt Romney padded his delegate count on the biggest night of the GOP presidential primary season but Rick Santorum demonstrated enough strength to ensure that there’s more convulsion ahead as Republicans struggle to settle on a candidate to take on President Barack Obama.

Super Tuesday gave Romney a narrow victory in pivotal Ohio, a home-state win in Massachusetts, and triumphs in Idaho, Vermont, Alaska, and Virginia. But it was no knock-out punch.

Santorum, for his part, won Oklahoma, Tennessee and North Dakota, and Newt Gingrich picked up his home state of Georgia.

“This was a big night tonight,” Santorum said. “We have won in the West, the Midwest and the South, and we’re ready to win across this country.”

The split decision refreshed questions about Romney’s appeal to conservatives in some of the most Republican states in the nation. The best-funded and best-organised of the four Republican candidates, Romney vowed to press on.

“Tomorrow, we wake up and we start again,” he told supporters. “And the next day, we’ll do the same. And so we’ll go, day by day, step by step, door by door, heart to heart.”

House Republican Leader Eric Cantor of Virginia, a Romney supporter, said Wednesday that Romney’s claim to the nomination is inevitable, adding that Santorum and Gingrich “have not demonstrated an ability to do what needs to be done.”

But in a morning interview on CBS “This Morning,” Cantor acknowledged there is still plenty of ongoing debate in “a robust party with many ideas.”