BERLIN (Reuters) - Postal voting for Germany's federal election began on Monday, piling pressure on conservative chancellor candidate Armin Laschet to reinvigorate his floundering campaign or else risk losing out to a left-leaning coalition.

       An official at the Federal Election Commissioner's office confirmed postal ballots would be sent out from Monday, six weeks ahead of the Sept. 26 election, after which Chancellor Angela Merkel plans to step down following 16 years in power.

       Merkel leaves behind a fractured political landscape in which no party enjoys a comfortable lead, and Laschet has seen his ratings slump since he was seen laughing on a visit to a flood-stricken town.

       Laschet's woes have opened up the possibility that the Social Democrats (SPD) could lead a three-way coalition with the Greens and the business-friendly Free Democrats (FDP), though FDP leader Christian Lindner has downplayed such a scenario.

       "A high share of postal ballots should benefit parties like the Greens, representing affluent, well-educated voters," said Carsten Nickel at Teneo, a political risk consultancy.

       "Postal voting requires quite some bureaucratic literacy and social capital. That poses huge hurdles for poorer and less knowledgeable groups."

       Pollsters and party strategists expect a third or more of voters to cast their ballot by post at this election as worries mount about a fourth wave of the coronavirus. At the last federal election in 2017, a record 28.6% of votes were by post.

       "We will experience a two-part election campaign," said Matthias Jung of pollster Forschungsgruppe Wahlen, as the parties seek to mobilise their supporters for the postal vote before trying to attract floating voters ahead of election day.

       Older people opt for postal ballots to a greater degree than younger voters, and Merkel's conservative alliance tends to do well among older voters, Jung added.

       Laschet is conscious that he needs to build momentum quickly as his conservatives have lost around 10 points in polls since the beginning of the year.

       "Our goal now is to become stronger in the next few weeks so that we can lead a future government," he said on Monday.

       The SPD is looking to maximise the popularity of its chancellor candidate, Olaf Scholz, who polls show voters much prefer to his rivals even if that has not yet translated into such strong support for the left-leaning party.

       (Writing by Paul Carrel; Editing by Alex Richardson)