The former Massachusetts governor had 52% of the vote with 68% of the towns holding caucuses reporting.
Undecided votes accounted for 2%.
The nonbinding votes, the first step toward
electing 18 Maine delegates to the Republican National Convention, took place in public schools, Grange halls, fire stations and town halls.
The Associated Press uses presidential preferences expressed in those caucuses to project the number of national convention delegates each candidate will have when they are chosen at Maine's state convention, calculating that Romney will wind up with all 18 delegates when all is said and done.
Campaigning in Minnesota, Romney noted that his victory in Maine came despite McCain's endorsement by the state's U.S. senators, both Republicans.
"The people of Maine joined those from across the nation in casting their vote for conservative change in Washington. . . . Like many Americans, the people of Maine are tired of Washington promises made but broken," Romney said in a statement.
Most of Maine's caucuses were held Saturday, but a scattering of towns will hold theirs today and later this month. Turnout was heavy, delighting party leaders. The showing was driven by the most hotly contested race in two decades, they said.
Besides Paul, who visited Maine on Monday, Romney was the only candidate to show up or send a surrogate to campaign as the caucuses drew close.
Romney's oldest son, Tagg, campaigned Friday and appeared at caucuses Saturday.
Romney has also won in the caucus states of Nevada and Wyoming, and finished first in Michigan's primary.
Kim Pettengill, a party activist for more than three decades, said Saturday's was the largest Kennebec County caucus turnout since 1980, the year Ronald Reagan won his first GOP nomination for president. Party Executive Director Julie A. O'Brien said other counties reported similarly heavy turnouts.
Overall, the Associated Press tracks the delegate races by projecting the number of national convention delegates won by candidates in each presidential primary or caucus, based on state and national party rules, and by interviewing unpledged delegates to obtain their preferences.
In some states, like Iowa, Maine and Nevada, local precinct caucuses are the first stage in the allocation process.
Maine Democrats hold their presidential preference votes at municipal caucuses on Feb. 10.