Twenty-eight percent of Republicans backed the former Massachusetts governor, giving him a lead of 8 percentage points over his nearest challenger Herman Cain in the poll, taken November 10-11.
Romney was 5 percentage points ahead in a survey November 7-8.
Newt Gingrich, the U.S. House of Representatives speaker in the mid-1990s, solidified a recent rise among conservatives seeking an alternative to the more moderate Romney, coming in third place in the current poll with 16 percent.
Gingrich, who is seen as having performed well in recent debates, was viewed as the second-most "presidential" of the Republican hopefuls, according to the poll.
Whether or not they support him, almost half of the Republicans surveyed expect Romney to become the nominee to oppose President Barack Obama, a Democrat, in the November 2012 election.
Romney, who also ran for president in 2008, has been in first or second place in polls for months and enjoys by far the most campaign funds of the Republican field.
But some in his party see him as too liberal, and he has so far failed to significantly boost his level of support in polls. The latest Reuters/Ipsos poll, however, shows he is benefiting from missteps by his rivals.
"As the other candidates falter, his image comes into relief," Ipsos pollster Cliff Young said.
The campaign of former pizza executive Cain has been dogged by allegations that he sexually harassed four women in the late 1990s. He has denied the allegations.
Texas Governor Rick Perry was in fourth place with 12 percent in the latest poll, which was taken on the heels of his disastrous performance in a debate on Wednesday when he could not remember key details of one of his main policy proposals.
He had 10 percent in the November 7-8 poll.
When judged on his personal qualities, Romney was rated strongly by Republicans, while both Perry and Cain trailed in important categories.
Romney, the former head of the Bain Capital private equity firm, says his business experience gives him an advantage over other Republicans and Obama in the quest to create jobs for the sluggish U.S. economy.
Far more voters see Romney as presidential than those who feel the same way about his Republican rivals, with 34 percent in the poll saying he is the most presidential candidate in the field.
"Going into the primaries, he's in a strong position and his image is solidifying around a very important attribute, which is being presidential, or being seen as presidential," Young said.
GINGRICH CHALLENGE
Gingrich, whose campaign is gaining momentum after struggling with staff desertions in the summer, was seen as the second most presidential candidate with 19 percent, 1 percentage point ahead of Cain. Perry trailed at 11 percent.
Gingrich, keeping to his form in previous debates, attacked Obama instead of the other Republican candidates at a debate devoted to foreign policy on Saturday in Spartanburg, South Carolina.
"There are a number of ways to be smart about Iran and relatively few ways to be dumb and the administration has skipped all the ways to be smart," Gingrich said, advocating covert operations to stop Tehran from making a nuclear bomb.
Perry's debate blunder last Wednesday, when he struggled to name the third of three government departments he would eliminate as president, has damaged his campaign but he remains in contention, the poll shows.
When shown the video of Perry's gaffe on Wednesday, 31 percent of poll respondents said he should withdraw from the campaign while just over half said he should stay in.
Romney was picked first when the Republicans were asked who would be "a strong leader for America," at 28 percent, with Cain following at 21 percent. Gingrich was at 20 percent and Perry trailed at 12 percent.
Although portrayed by both Democrats and Republicans as someone who changes his stance for political purposes, Romney fared better than Cain and Perry when respondents were asked which candidate "will say anything to win votes."
Twenty-one percent chose Cain, 20 percent pointed to Perry, Romney was at 19 and Gingrich came in at 8 percent.
When asked which candidates were "too radical to lead America," the Republicans in the poll put Cain at the head of all the candidates with 21 percent and Romney last at 8 percent. Gingrich had 12 percent and Perry was at 11 percent.
Cain and Romney tied at 24 percent when the voters were asked who has the best solutions for U.S. economic problems, with Gingrich at 15 percent and Perry at 12 percent.
When asked who "understands the problems of someone like me," the Republican voters failed to give any candidate more than 25 percent support. Cain was highest at 25 percent, to Romney's 16 percent, 15 percent for Perry and 14 percent for Gingrich.
The poll results are from an online survey of 461 Republican registered voters.
Because this was an online poll, typical margins of error do not apply. Despite that, various recognized methods were used to provide a representative sample and weighted results. If this were a traditional random survey, it would have a margin of error of 4.5 percentage points.
(Editing by Alistair Bell and Paul Simao)
Mochila insert follows.
