PARIS -- Emmanuel Macron saw off the far right's Marine Le Pen in April's presidential election, but now the French president is facing a threat from the other end of the political spectrum in the fight for parliamentary power.

Elections are being held nationwide to select the 577 members of the most powerful branch of France's Parliament, the National Assembly, on Sunday, with a runoff on June 19.

The far-left's Jean-Luc Melenchon, the head of a coalition made up of leftists, greens and communists under the name Nupes, is seeking to win the election and prevent Macron's party from retaining its current parliamentary majority.

Former Trotskyist Melenchon wants to significantly increase the minimum wage and lower the retirement age to 60. He also wants to be prime minister if his coalition gains control. That scenario would have the power to derail Macron's domestic agenda.

Here's a closer look:

WHAT'S MACRON GOT TO LOSE?

A lot. If Macron's coalition, Ensemble!, keeps control, the president will be able to carry out his agenda as before. But observers think Macron's party and allies could have trouble getting an absolute majority -- the magic number of 289 seats -- this time around.

A government with a large, but not absolute, majority will still be able to rule, but only by bargaining with MPs.

Though Melenchon's coalition could win more than 200 seats, current projections give the left little chance of winning a majority. Macron and his allies are expected to win between 260 and 320 seats, according to latest polls.

Macron will have substantial powers over foreign policy whatever the outcome of this poll. But a poor showing for his coalition could be a thorn in his side for the rest of his second five-year term. This could prove disastrous for the president's agenda, which includes tax cuts, welfare reform and raising the retirement age.

"If Macron loses control, he will be a hobbled president -- mainly in charge of foreign affairs, defence and Europe. But key economic and domestic issues will be decided by the government. And if it's Melenchon's coalition it will be very hostile to his agenda," said Olivier Rozenberg, associate professor in legislative studies at the Sciences Po university.

COHABITATION

The last time France had a president and parliamentary majority from different parties was two decades ago, when conservative President Jacques Chirac found himself working alongside a Socialist prime minister, Lionel Jospin. This fraught power-sharing scenario is called cohabitation. To try to avoid this and prevent gridlock, in 2000 the constitution was changed to reduce the presidential term from seven to five years and move the parliamentary elections into the same five-year cycle.

But this year's vote is closer that it's been in years. If Nupes gains control, Macron would be forced to name a prime minister from that coalition.

"Prime Minister (Elisabeth) Borne will be forced to quit, all ministers will change, and will be chosen by the prime minister. Probably a Prime Minister Melenchon," said Rozenberg. "Difficult is not even the word."

MELENCHON: THE COLORFUL RADICAL

Coming in a strong third placing in April's presidential election, the mercurial septuagenarian leader of the France Unbowed party pressed on to capitalize on this popularity.

He has a radical vision for France -- and a theatrical way of presenting it. Finance Minister Bruno Le Maire branded him the "Gallic Chavez," comparing him to the late Venezuelan president.

Melenchon recently softened his tone to attract more traditional leftist voters, and took a resolutely pro-environment stance, bringing him growing support from young people.

Melenchon would like the Fifth Republic, established by Charles de Gaulle in 1958, scrapped in favour of a Sixth Republic with the aim of making it more democratic and parliamentary-based, instead of the current presidential system. He also wants to lower the retirement age to 60, restore wealth taxes and raise the minimum wage by 15%.

HOW IT WORKS

The French system is complex and not proportionate to the nationwide support for a party. Legislators are elected by district.

A parliamentary candidate requires over 50% of the day's vote to be elected outright on June 12.

Failing that, the top two contenders, alongside anyone who won more that 12.5% of the registered vote, go forward to a second round. In some cases, three or four people make it, though some may step aside to improve the chances of another contender.

That tactic has often been used to block candidates from Marine Le Pen's far-right National Rally party.

Le Pen's party, which won eight seats in 2017, hopes this time to get at least 15, allowing it to form a parliamentary group and gain greater powers at the Assembly.

The National Assembly, the lower house, is the more powerful of France's two houses of parliament. It has the final say in the law-making process over the Senate.

ISSUES AT STAKE

Inflation is a key issue among voters, as energy and food prices soar. Macron hopes his initiatives to boost growth and food production will play well among the electorate. However, Melenchon's plans to raise the minimum wage to 1,400 euros a month will surely curry favour among blue-collar voters.

Police violence has also become a political hot potato recently after a fatal police shooting in Paris. That came a week after police chiefs were condemned for using tear gas on soccer fans at the Champions League final in the French capital. The left has capitalized on the incidents to criticize Macron over brutal policing methods. Still, observers say, Macron does well in voters' eyes on security issues, as he has struck a harder line than the left historically.

"Macron is more credible in terms of security. A silent majority of the population look to him to lead on this issue," said Rozenberg. "This could play into his hands."

Another factor that could benefit Macron is the predicted high rate of abstention.