Mitt Romney won Republican primary contests in Arizona and Michigan, maintaining his front-runner status.
In Michigan, the state his father governed and where he was raised, Romney beat back a challenge Tuesday by Rick Santorum, the former Pennsylvania senator, 41 percent to 38 percent, with nearly all of the vote counted.
But Santorum’s strong challenge forecasts a long and difficult fight for Romney to win the nomination for president. The former Massachusetts governor had to outspend Santorum in a state that just weeks ago he had been expected to win handily.
Romney won handily in Arizona, defeating Santorum by 47 percent to 27 percent.
Michigan, with its battered automaker-based economy and its status as a large Midwestern hub, was considered a critical test.
The candidates now will focus on Super Tuesday, when 10 states vote on March 6.
Santorum has surged to become the likeliest conservative contender to beat Romney by playing up his blue-collar roots and emphasizing social conservatism on issues such as birth control, abortion and gay rights.
Newt Gingrich, the former U.S. House of representatives speaker, scored 6.5 percent in Michigan and 16 percent in Arizona, and Rep. Ron Paul (R-Texas) scored 12 percent in Michigan and 8 percent in Arizona.