|Romney's victory on Saturday is his fourth campaign triumph in a row [Reuters]
Republican Mitt Romney has won the Washington state presidential caucus, in a boost to his campaign heading into next week's "Super Tuesday" contests in 10 states.
Rick Santorum and Ron Paul battled for second place, while Newt Gingrich ran a distant fourth.
Romney's victory on Saturday came on the heels of twin primary triumphs over Santorum earlier in the week in hard-fought Michigan and lightly contested Arizona, as well as a narrow win over Paul in Maine caucuses earlier in February.
Returns from caucuses in 43 per cent of Washington State’s precincts showed Romney with 37 per cent of the vote, while Paul and Santorum each had 24 per cent. Gingrich was drawing 11 per cent.
There were 40 delegates at stake, and a likelihood that at least two of the contenders for the nomination to oppose President Barack Obama would add to their totals.
Romney, the former Massachusetts governor, had 173 delegates at the beginning of the day, according to an Associated Press count that includes party officials who will vote on the selection of a nominee but are not selected at primaries or caucuses. Santorum had 87, Gingrich 33 and Paul 20. It takes 1,144 delegates to win the nomination at the Republican National Convention this summer in Tampa.
The Republican race has shared the political spotlight in the past few days with a controversy in which conservative talk show host Rush Limbaugh called a Georgetown University law student a "slut" and a "prostitute" — an issue that the GOP presidential rivals seemed reluctant to comment on.
Limbaugh apologised on his website during the evening to the woman, Sandra Fluke, who had spoken out publicly in favour of a requirement for most insurance coverage to include contraception.
Romney, Santorum and Gingrich were all campaigning in Ohio — the most intensely contested of the Super Tuesday states — as the first caucus returns were reported.