Mitt Romney won Republican primary contests in Arizona and Michigan, maintaining his frontrunner status for now.
In Michigan, the state his father governed and where he was raised, Romney, the former Massachusetts governor, beat back a challenge Tuesday by Rick Santorum, the former Pennsylvania senator, 41 to 38 percent, according to projections with 84 percent of the vote in.
Santorum's strong challenge nonetheless forecasts a long and difficult primaries fight for Romney, who had to outspend Santorum in a state he had just weeks ago been expected to win handily.
Michigan, with its battered automaker-based economy and its status as a large Midwestern hub, was considered a critical test.
The candidates will now focus on the "Super Tuesday" contest, when ten states vote on March 6.
Santorum has surged to become the likeliest conservatiive contender to beat Romney by playing up his blue collar roots and emphasizing social conservatism on issues like birth control, abortion and gay rights.
Romney did better in Arizona, besting Santorum by 48 to 26 percent with 55 percent of the polls reporting.
Newt Gingrich, the former U.S. House of representatives speaker, scored 7 percent in Michigan and 16.5 percent in Arizona, and Rep. Ron Paul (R-Texas) scored 12 percent in Michigan and 8 percent in Arizona.