PARIS—France’s far-right National Front has struggled to raise funds at home to finance leader Marine Le Pen’s presidential bid in May and is seeking loans from banks abroad, party officials say—something opponents say opens the door to foreign influence.

Ms. Le Pen is running at the head of the election pack in public-opinion surveys. But French lenders are unwilling to extend credit to the National Front because they disagree with the anti-immigrant, euroskeptic party’s policies, party treasurer Wallerand de Saint Just said.

In France, the government covers a large portion of political parties’ campaign expenses, but it can take several months for it to actually hand over the money, forcing candidates to get stopgap loans from banks.

Mr. de Saint Just said that unless domestic lenders change their minds or supporters step up with bigger donations, the National Front would need to seek loans abroad—from banks elsewhere in Europe, the U.S. or Russia. “For the moment, we haven’t found a real solution,” he said.

In 2014, the National Front borrowed about €9 million ($9.6 million) from the now-defunct Moscow-based First Czech-Russian Bank to help fund its campaign for seats in the European Parliament, drawing criticism from rival politicians who said the loan was a sign of undue Kremlin sway over the National Front.

First Czech-Russian Bank was declared bankrupt and taken over by Russia’s deposit-insurance agency last year.

The National Front and Ms. Le Pen have long espoused pro-Russian views and expressed admiration for President Vladimir Putin. Earlier this year, Ms. Le Pen said the annexation of Crimea wasn’t illegal and criticized the Western sanctions levied against Moscow.

“We need to prevent foreign powers from influencing our elections,” said Romain Colas, a French Socialist lawmaker, who proposed legal changes to require political parties to disclose the identities of their bankers and the terms of their loans.

The proposed legislation was struck down by France’s Constitutional Council in December for procedural reasons.

Mr. Colas said France needed to be especially cautious about Russia. The U.S. government has said Russia hacked into computers belonging to the Democratic Party and leaked information it obtained ahead of the U.S. elections in November.

According to a survey conducted by polling firm Ipsos Sopra Steria, Ms. Le Pen is favored by 25% to 26% of voters. The Républicains’ presidential candidate, François Fillon , would receive 24% to 25% of votes and Emmanuel Macron—the former economics minister who quit President François Hollande’s government and launched his own political party—19% to 21%, Ipsos found.

Political parties in France aren’t required to publish all the financial details of their campaigns. Expenses, however, are capped at around €17 million in the first round of a presidential race. Candidates who make it to the second round can spend up to €22.5 million in total.

If a politician exceeds a vote threshold of 5%, the government will reimburse expenses of up to €8 million for the first round and €10 million in total.

Several banks in France, including Société Générale, which lent €4 million to Ms. Le Pen for her presidential bid in 2012, recently chose to stop lending to political parties and their candidates.

“Société Générale’s credit policy no longer allows lending to a political party, whichever it may be, for economic reasons but also to ensure political neutrality,” a bank spokeswoman said.

The National Front said other banks have refused to consider loan applications from Ms. Le Pen.

“I doubt that’s true,” said Jean-François Debat, treasurer of the rival Socialist Party. “They’re just crybabies.”

Mr. Debat said the Socialists have arranged loans from a domestic bank. Hard-left candidate Jean-Luc Mélenchon has also lined up bank loans, according to officials of his party.

Gilles Boyer, campaign treasurer for the center-right Mr. Fillon, and Sylvain Fort, a spokesman for Mr. Macron, say the two men are on track to get loans from French lenders.