By William Horobin

PARIS--Emmanuel Macron's presidential campaign received a boost Wednesday after the leader of a centrist political party with a small but ardent following said he wouldn't run in the election and proposed an alliance with the former French economy minister.

François Bayrou, leader of Mouvement Démocrate, said contesting this year's election would have fractured the vote to the advantage of Marine Le Pen, the leader of far-right National Front party.

"Because the risk is immense, because French people are bewildered and desperate, I've decided to offer Emmanuel Macron an alliance," Mr. Bayrou said.

Mr. Bayrou's decision is the latest twist in a closely fought presidential election, fraught with setbacks for front-runners and surges in support for outsiders. With polls showing Ms. Le Pen winning the first round April 23, mainstream candidates are jostling for second place and the chance to go head-to-head with the National Front leader in the May 6 runoff.

The campaign of center-right candidate François Fillon--once clear favorite to win the election--was rattled by the launch of an investigation in January into allegations that his wife collected a state salary without working.

Mr. Macron, who set up his political party En Marche less than a year ago, overtook Mr. Fillon in the polls this month, but his support has waned in recent days after he made critical comments of France's colonial past.

A tracking poll of 1,386 people by Ifop Tuesday showed Mr. Fillon and Mr. Macron tied on 19% in the first round of a presidential election, behind Ms. Le Pen on 26%. Ms. Le Pen would lose the second round vote against either candidate, although by a narrowing margin, according to recent polls.

If Mr. Bayrou's voters transferred wholesale to Mr. Macron, he could gain as much as six points, polls show. Both men have crafted brands as centrists from outside of the establishment, seeking to undermine the dominance of France's Socialist Party and Mr. Fillon's center-right Les Républicains party.

"It's a real boost for Mr. Macron. Mr. Bayrou can suck in the centrist votes," said Ifop analyst Jérôme Fourquet.

The alliance between Mr. Macron and Mr. Bayrou reassured investors who were spooked in recent days by the strengthening of Ms. Le Pen in the polls. As Mr. Bayrou announced his plans, investors piled into French debt and the spread between French and German 10-year sovereign debt tightened to 0.74 percentage points from 0.826. The CAC-40 equity index rose to 4,899 from 4,874.

Officials at En Marche later said Mr. Macron accepted Mr. Bayrou's offer and the two men will meet Thursday to discuss mechanics of the alliance.

Mr. Macron on Wednesday also received the backing of François de Rugy, an ecologist candidate who ran in January's leftist primary but said he wouldn't support the Socialist winner of the vote, Benoît Hamon

"It is clearly a turning point in the election campaign," said Gérard Collomb, Socialist mayor of Lyon and supporter of Mr. Macron.

--Mike Bird contributed to this article.

Write to William Horobin at william.horobin@wsj.com

(END) Dow Jones Newswires

February 22, 2017 14:17 ET (19:17 GMT)

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