The former Massachusetts governor has won in Nevada.
Preliminary results from the western state's caucases show Romney with about 48 percent of the vote. That's about twice as much as his closest rival, Newt Gingrich, a former House speaker, and Ron Paul, a libertarian senator.
The victory was Romney's second in a row and his third in the first five contests in the state-by-state battle to find a Republican challenger to President Barack Obama.
Analysts say the latest victory gives Romney a big wave of momentum as he heads into the next contests -- in Minnesota, Colorado, and Missouri on February 7.
Compiled from agency reports