On Tuesday, the centrist Mr. Macron also received the unexpected backing of Yanis Varoufakis, Greece’s former finance minister in the government of Prime Minister Alexis Tsipras and a darling of the political left.

Mr. Varoufakis, writing in an op-ed for Le Monde, said that “French progressive voters have all the reasons to be angry against Emmanuel Macron” because of his economic policies, but he said it was crucial to keep Ms. Le Pen from winning power. Mr. Varoufakis also praised Mr. Macron for personally reaching out to him at the height of Greece’s debt crisis in 2015, to try to reopen talks.

“I think it is my duty to ensure that French progressives, who are about to enter (or not enter) the voting booth in the second round of the presidential election, be fully aware of this as they make their choice,” he wrote.

Many hard-left voters have been put off by Mr. Macron’s economic policies and his support for labor regulation overhauls when he was economy minister. So much so that the France Unbowed movement of Jean-Luc Mélenchon, the hard-left candidate who received 19.6 percent of the first-round vote, announced that its members were not inclined to turn out for Mr. Macron on Sunday.

In an unscientific online survey of Mr. Mélenchon’s supporters conducted by his party, only 34.8 percent said they would vote for Mr. Macron against Ms. Le Pen. Nearly two-thirds of those who participated in the poll said they would abstain or cast a blank ballot in the final round.

Though more than seven million people voted for Mr. Mélenchon in the first round, fewer than 250,000 people took part in the online straw poll. Voting for Ms. Le Pen was not one of the options.

Ms. Le Pen may have an equally difficult time, no matter whose words she uses, persuading the supporters of her vanquished rivals to back her candidacy. Damien Abad, a former spokesman for Mr. Fillon, said that she was unlikely to persuade many of those who had voted for Mr. Fillon. “François Fillon’s voters aren’t fooled,” he told BFM-TV. “They won’t be bought because one copies parts of their candidate’s speech.”