Mitt Romney has pulled off a double win in the Michigan and Arizona primaries.
Mr Romney is vying to be the Republican Party's candidate against President Barack Obama in the election in November this year.
In Arizona, he leads with 47% of votes, and heads Rick Santorum in Michigan by 41% to 38% with 99% counted on Tuesday.
Michigan was seen as vital for Mr Romney. He was born in the state where his father was governor, but had struggled to win over conservative voters, the BBC reports.
Mr Santorum addressed supporters in Michigan, moments after he reportedly called Mr Romney to concede victory, thanking voters for their support.
Former House Speaker Newt Gingrich and Texas Congressman Ron Paul did not compete in Arizona and Michigan, focusing on contests scheduled for next week.
The wins in Michigan and Arizona give Mr Romney's campaign new momentum ahead of next week's crucial contests in 10 states, known as Super Tuesday.
Mr Romney earlier won contests in New Hampshire and Florida, but had struggled in recent weeks.
He currently has 152 delegates, compared to Mr Santorum's 72, with 1144 needed to secure the nomination.