WASHINGTON (Reuters) - Despite his strong showing in early state contests in the race for the Republican U.S. presidential nomination, Mitt Romney's support nationwide has dipped slightly during the past month, according to a Reuters/Ipsos poll released on Tuesday.
Romney was backed by 29 percent of Republican voters in the telephone poll conducted February 2-6, down from 30 percent in a survey in early January, although the change was within the poll's margin of error.
The results suggest Romney - despite his vast advantages in organization, fundraising and momentum after victories in New Hampshire, Florida and Nevada - still has many doubters among Republicans nationwide.
"He still hasn't really convinced all the Republicans across the country that he's the guy to get behind," said Chris Jackson, research director for Ipsos public affairs.
The former Massachusetts governor's three rivals in the race to oppose Democratic President Barack Obama in the November 6 U.S. election were in a virtual tie for second, the poll showed. The gaps between the three were within the poll's margin of error.
Support for Ron Paul, a U.S. congressman from Texas, grew by 5 percentage points to 21 percent. That moved him into second place and ahead of former House of Representatives speaker Newt Gingrich, whose support slipped to 19 percent from 20 percent.
Support for Rick Santorum, a former U.S. senator from Pennsylvania, rose by 5 percentage points to reach 18 percent, putting him just behind Gingrich, according to the poll.
Republican strategist Doug Heye said Romney's nationwide poll numbers were being held down partly because he is the front-runner in the Republican race, making him a target for his rivals.
"The other candidates are all piling on Romney," Heye said, "and that's going to have an effect."
The Reuters/Ipsos poll also showed Obama's approval rating ticking upward during the past month, a period in which favorable reports on the economy seem to have given his re-election effort some momentum.
Obama's approval rating in the new poll was 48 percent, up from 47 percent in January. His disapproval rating was higher, though: 49 percent, unchanged from last month.
"American popular opinion is not really changing abruptly," Jackson said. "Obama's in a good position, but ... this race could become much more competitive very fast if the economic news does not continue to be positive."
The U.S. unemployment rate in January was 8.3 percent, down from 8.5 percent in December. A jobs report last Friday was unexpectedly strong, and appeared to boost claims by Obama and his re-election campaign that the economy, although still shaky, was continuing to recover.
The poll offered some encouraging signs for Obama.
In a survey of registered voters, Obama led Romney, 48 percent to 42 percent, in a head-to-head match-up, a slightly larger margin than last month.
With Gingrich as the opponent, Obama led 50 percent to 38 percent - down slightly from the 52 percent to 38 percent advantage Obama had in the January poll.
In the Republican race, many conservatives have worried that Romney is not sufficiently conservative to be the Republican nominee to face Obama.
Gingrich, Santorum and Paul both have tried to stake a claim as the conservative alternative to Romney.
Gingrich and Santorum both have had shining moments during the state-by-state race for the Republican nomination. Paul, known for his libertarian views, has shown the ability to consistently attract a core of loyal supporters.
Santorum won the Iowa caucuses on January 3 by a razor-thin margin over Romney, and Gingrich shocked Romney with a 12 percentage point victory in South Carolina on January 21.
For the most part, however, the campaign has been dominated by Romney - and punctuated by attacks on Gingrich's character by Romney's campaign and an independent "Super PAC" that supports the former Massachusetts governor.
Romney has a strong organization and is maintaining a lead that is likely to build over time, Heye said, boosted in part by the Gingrich and Santorum campaigns' lack of a broad organization.
Neither Gingrich nor Santorum submitted enough voter signatures to get on the March 6 primary ballot in the key state of Virginia, for example.
"People tend to forget that," Heye said.
Paul has not come close to winning any of the first five states to vote in the Republican nomination process, and has stumbled to fourth-place finishes in the hotly contested contests in South Carolina and Florida.
But Paul's campaign has built a following among those who support his calls for dramatically lower taxes, a $1 trillion cut in the U.S. budget and much less U.S. military involvement overseas.
"This poll further illustrates that Ron Paul is emerging as the real conservative alternative to Mitt Romney," Paul spokesman Jesse Benton said in a statement.
The Reuters/Ipsos telephone poll of 1,033 adults included 881 registered voters, of whom 503 were Democrats, 405 Republicans and 125 independents.
The margin of error was plus or minus 3.1 percentage points for all respondents; 3.3 points for registered voters; 4.4 points for Democrats; 4.9 points for Republicans; and 9.6 points for independents.
Editing by David Lindsey and Will Dunham