Portraits of the candidates who will run in the second round in the 2017 French presidential election, Emmanuel Macron (L), head of the political movement En Marche !, or Onwards !, and Marine Le Pen, French National Front (FN) political party leader. Reuters

Emmanuel Macron's assumed grip on the keys to the Elyee Palace could be loosening as his lead against far-right Marine Le Pen has narrowed ahead of the final week of the French Presidential race. The centrist candidate's predicted win dipped below 60 percent for the first time since mid-March, according to the latest Opinionway poll, which saw Le Pen's campaign gaining further traction after she temporarily distanced herself from her Front National party. The polls now see Macron emerging victorious from May 7's second round runoff, but with 59 percent to Le Pen's 41 percent – a notable shift from his anticipated 65 percent clear lead. However, it is still very much all to play for. The polls have been wrong before, however, and the final outcome will largely hinge on the decision of the almost 55 percent of the population who did not back either candidate. Voter turnout crucial It is anticipated that left-leaning voters could swing to back Macron while more conservative members of the electorate could be inclined to back Le Pen; however, a large proportion of French people say they will abstain entirely. Approximately 35 percent of Jean-Luc Melenchon backers, 24 percent of Francois Fillon supporters, and around 20 percent of Benoit Hamon voters say they will not vote, disheartened as they are by their choice, according to a composite of polls taken after the first round.

Add to this the 22 percent of potential voters who did not turn out in the first round and the outcome is uncertain. "Turnout is important as Le Pen's best bet to win the election is for voters who did not support Macron in the first round to stay home on Election Day," Antonio Barroso, managing director at Teneo Intelligence, told CNBC via email. While turnout would have to be exceptionally low for Le Pen to have a "real shot," said Barroso, he noted that allegiances do not seem to have moved significantly since the first round. "Rather, we are seeing an increasing number of Fillon and Melenchon voters that are saying they will stay home on 7 May." 'No' votes could swing it Indeed, there are a growing number of people campaigning for abstention. In the wake of the first round result, a number of French voters took to Twitter to campaign to boycott the election under the hashtag #SansMoiLe7Mai. "There is a very big complacency risk in the second round, with voters either not bothering to turn out or spoiling their ballots in protest," a Eurasia note said earlier this week. This has also prompted a number of social groups, such as Strasbourg's Citizen Challenge, to promote voting in areas of the country with particularly low turnout. "The game is changing," Edouard Lecerf, global director of political & opinion research at Kantar Public, told CNBC. "If, for some reason, some of (Macron's) potential voters (those coming from other first round candidates) prefer not to go and vote (they would not cast a vote for Le Pen)… and if on the contrary, Le Pen gets the largest proportion of those who did not turn out at the first round (22 percent of potential voters) but come to join the game for the second round, the game is changing…"

Supporters of French presidential election candidate for the En Marche ! movement Emmanuel Macron cheer following the announcement that he was on course to qualify for the run-off in France's presidential election in Paris on April 23, 2017. Patrick Kovarik | AFP | Getty Images