Momentum for the opposition has been gathering, and experts believe that will narrow the margin of victory for Najib.
Consulting firm Eurasia Group says there's an 85 percent chance that Najib will win.
Research firm Capital Economics said in an April note that Najib's chances will be improved by "years of gerrymandering" that will make it easier for the government to collect a majority. It also claimed that "wildly" different sizes of parliamentary constituencies meant it was much easier for him to win a majority.
In response, a Malaysian government spokesperson told CNBC that the redrawing of electoral boundaries to account for population growth is a "perfectly normal practice" that has been applied in democracies around the world.
"In Malaysia, these changes were proposed and implemented by the independent Election Commission and subsequently approved by the judiciary, whose impartiality is evidenced by the fact that it frequently rules against the government and senior ministers," the statement said.
A survey released by pollster Merdeka Center last week showed the opposition making gains, but not enough to land a majority of parliamentary seats.
Mahathir's Pakatan Harapan coalition is likely to win 43.7 percent of the popular vote, while Najib's coalition is expected to draw 40.3 percent, Merdeka Center said last week.
Under Malaysia's system, the party that gets the most seats in parliament wins — even if it does not win the popular vote.
—Reuters contributed to this report.