Mitt Romney has scored a hard-fought triumph in his native state of Michigan and powered to victory in Arizona, dealing a blow to his chief rival and gaining precious momentum in the turbulent Republican presidential race.
Mr Romney's victories cement his status as his party's front-runner to challenge President Barack Obama in the November election, and injects his campaign with new energy ahead of next week's crucial contests in 10 states, known as Super Tuesday.
Rick Santorum had needed a win or a very close second to show that he was still in play and that victories earlier this month in Minnesota, Colorado and Missouri were not a fluke.
Mr Romney is viewed as the candidate best positioned to beat Mr Obama, and he has the backing of much of the Republican establishment.
But Mr Santorum has captured the heart of the party's conservative base, which has no doubt about the authenticity of his views on social issues such as abortion and considers Mr Romney too moderate.
"We didn't win by a lot but we won by enough, and that's all that counts," Mr Romney told cheering supporters.
Mr Santorum said he would continue his presidential bid, pushing an economic message.
With 87% of Michigan's precincts reporting, Mr Romney had 41% to Mr Santorum's 38%. Texas congressman Ron Paul was in third place with 12%, and former House of Representatives speaker Newt Gingrich was bringing up the rear with 7%.
In Arizona with 62% of precincts reporting, Mr Romney was leading with 48% to Mr Santorum's 26%. Mr Gingrich was third with 16% and Mr Paul came in last with 8%.
Mr Romney's victory in Arizona had been expected, so much so that his opponents spent little time and no money campaigning there.