Many analysts said a win in Maine was crucial for Mr Romney’s campaign
Officials in Maine say Mitt Romney has won the state’s primary caucuses in the race for the Republican presidential nomination with 39% of the vote.
With most of the votes counted, libertarian outsider Ron Paul is in second place with 36%.
Mr Romney’s camp is hoping victory in Maine can help his campaign recover after conservative rival Rick Santorum won polls in three states on Tuesday.
The fourth candidate, Newt Gingrich, did not campaign hard in Maine.
Mr Santorum, now viewed as Mr Romney’s main rival nationally, also had only limited campaign activity in the state.
Mr Paul told his supporters not to be discouraged and said the outcome had been “a virtual tie”, AP reports.
The caucuses will not automatically decide which candidate receives Maine’s delegates at the Republican National Convention in August, and the poll is largely viewed as a beauty contest, reports the BBC’s Jonathan Blake in Washington.
However, Mr Romney still needed this win to avoid serious questions about his campaign, our correspondent adds.
This result will provide his candidacy with added momentum ahead of contests in Arizona and Michigan at the end of the month, he adds.
Mr Romney has previously served as governor of nearby Massachusetts and won the Maine caucuses by a much wider margin in his failed 2008 bid for the presidential nomination.
‘Heartened’Earlier Mr Romney had received a boost at a major gathering of conservative activists in Washington.
He topped a straw poll held at the end of the three-day Conservative Political Action Conference, with 38% of the vote.
Mr Santorum came in second with 31%, with Mr Gingrich and Mr Paul on 15% and 12% respectively.
Mr Romney tweeted that he was “honoured” to have won the poll.
“I’m heartened that so many friends here agree with me about the need for conservative change”, he said.
While the candidates had previously focussed during the early weeks of the primary season on only one or two states at time, the upcoming schedule will see the Republican hopefuls scatter across the country in search of strategic wins.
On 6 March, popularly known as “Super Tuesday”, 11 states will hold contests.
Mr Paul is targeting those states, where he believes his organisation and enthusiastic volunteer base can win him delegates.