Conservative voters, particularly the elderly, are still wary of the National Front (FN) which they view as too radical.
To stand a chance of winning, she will need to convince people like pensioner Jacques Villain and student Marina Campana ahead of the final round of the presidential election on May 7.
Both of them backed defeated candidates in the first round last weekend and now face a choice between Le Pen and pro-European centrist Emmanuel Macron in the run-off.
Villain, a retired pensioner in Nice, supported defeated conservative Francois Fillon while Campana, a 19-year-old who attends university in the southern city, voted for far-left firebrand Jean-Luc Melenchon.
But both ruled out a vote for Le Pen, saying the main reason was her association with racism which she has attempted to erase since taking over the National Front (FN) in 2011.
Villain, out for a walk on the famed waterfront promenade in Nice that was the scene of a deadly Islamist-inspired truck rampage last July, said he would vote for Macron out of duty, not conviction.
"The National Front is not a normal party," he said. "It brings back bad memories" of France's past.
Campana told AFP ahead of a campaign rally by Le Pen in Nice on Thursday : "There are things she says on immigration and security and I just don't agree."
Their views are far from universal -- some voters encountered in the traditionally right-wing city said they would switch to Le Pen -- but there remains wariness among a large part of the electorate.
This explains in part why polls show Le Pen would lose the second round by a large margin, 40 percent to 60 percent for Macron, if it were held today.
One in four Fillon supporters and fewer than one in five Melenchon voters currently plan to vote for her, according to an Ifop poll released Thursday.
This dynamic gives the 48-year-old Le Pen a tricky balancing act ahead of the runoff: keeping her core far-right supporters happy, while appealing to a wider electorate who need to be reassured.
She gave a hint of this on Thursday evening as she addressed the rally in Nice that mixed attacks on Macron -- the candidate of the banks and "oligarchs" -- with messages to moderate voters.
The tone was markedly different from her speeches of the last fortnight which dwelt extensively on immigration, the threat from radical Islam and the perceived loss of French identity.
She quoted Jean Jaures -- a famed former Socialist leader beloved on the left -- and namechecked Charles de Gaulle, the founder of the French republic and hero on the right.
"I don't look at your origins, your religion, your sexual orientation, or the colour of your skin. That doesn't interest me. What interests me is you," she said at one point.
Le Pen also addressed anxiety among voters about her plans to pull France out of the European Union and replace the bloc with a club of independent nations.
"I've heard the fears expressed," she said, explaining how she would invite European leaders immediately after her election to talks to repatriate powers to national capitals.
The former lawyer would then call a referendum on the deal. "Nothing will be done without you, nothing will be done against you," she said.
Notably absent was her proposal to withdraw France from the eurozone and return to the franc -- one of her signature policies backed by only a minority of voters, polls show.
Herve Le Bras, a veteran sociologist and expert on the far-right in France, says conservative voters, particularly the elderly, are still wary of the FN which they view as too radical.
"They are in favour of changing things, but not for turning the table upside down," he told AFP.
He said he thinks Le Pen was disappointed by her first-round score of 21.3 percent and says the FN has progressed slightly nationwide, "but it's not a tidal wave".
"I think she was very disappointed and is hoping to make up for it with a bigger score in the second round," he said.