BERLIN, Sept 7 (Reuters) - Chancellor Angela Merkel made an impassioned plea to German voters on Tuesday to back her would-be successor Armin Laschet at this month"s national election, as an opinion poll showed support for their conservatives slumping to an all-time low. The Forsa poll for RTL/n-tv put support for the centre-left Social Democrats (SPD) at 25%, extending their lead over the conservative CDU/CSU bloc, who dropped 2 points from the previous week to 19%, which n-tv said was a record trough. The SPD only took a poll lead last month, an upset that has blown wide open the election to determine the course for Germany, Europe"s largest economy and most populous country, after 16 years of steady, centre-right leadership under Merkel. She plans to step down after the poll. "Citizens have the choice in a few days: either a government that accepts the support of the (far-left) Linke party with the SPD and the Greens, or at least does not exclude it," Merkel told lawmakers in the Bundestag lower house of parliament. "...or a federal government led by the CDU and CSU with Armin Laschet as chancellor - a federal government that leads our country into the future with moderation," she added, in what was likely her last speech to the chamber. After losing their lead in polls, the conservatives are increasingly trying to revive their struggling campaign with warnings of a lurch to the left under an SPD-led coalition. The far-left Linke pitched themselves on Monday as would-be coalition partners for the SPD and Greens, both of whom would be uncomfortable with such a red-green-red alliance. The SPD"s candidate for chancellor, Olaf Scholz, has repeatedly distanced himself from the Linke, calling the party unfit for government as long as it does not clearly commit to solid public finances and the NATO military alliance, the transatlantic partnership with the United States. But, without naming the party in a later televised town hall event, he said it was not for him to decide ahead of the election who he should govern with, since that was up to voters. "Now is the hour of the citizens. They will decide, and the stronger the SPD vote, the better the chances for a good government," he told ARD television. Merkel said Laschet would lead a government that stands for "stability, reliability, moderation and the middle ground - and that is exactly what Germany needs". But Laschet"s promise of "steadfastness" is failing to resonate with voters worried about climate change, immigration and the COVID-19 pandemic.