BERLIN — Outgoing Chancellor Angela Merkel’s center-right bloc is kicking off its official campaign for Germany’s Sept. 26 election amid a worrying sag in poll ratings and increasing criticism of Merkel’s would-be successor, Armin Laschet.

Laschet is set to speak at a Saturday rally along with Merkel, who has largely stayed out of the campaign so far. The rally in Berlin comes as recent polls have shown support for the Union bloc slipping as low as 23% — leaving it only a few points ahead of the center-left Social Democrats and the environmentalist Greens.

The polls also have shown dismal personal popularity ratings for Laschet, the governor of Germany’s North Rhine-Westphalia state and the leader of Merkel’s Christian Democratic Union party, even as Social Democratic rival Olaf Scholz — the vice chancellor in Merkel’s coalition government — has gained ground.

Merkel, who has led Germany since 2005, announced in 2018 that she wouldn’t seek a fifth term as chancellor. The Union took 32.9% of the vote in the last election, in 2017. In its best result under Merkel, in 2013, the bloc won 41.5%.

Also set to speak at Saturday’s campaign rally is Markus Soeder — the head of the CDU’s Bavarian sister party, the Christian Social Union. Soeder battled Laschet in April for the nomination to run for chancellor and succeed Merkel.

While Laschet has declined to comment on the recent poll ratings, Soeder has shown signs of impatience with the Union’s campaign. He said last month that the bloc needs to show it isn’t trying to “travel to the chancellery in a sleeping car.”

Laschet is a centrist figure in Merkel’s mold but doesn’t appear so far to have inspired voters or to have impressed people with his management of the severe floods that hit his state last month. On Wednesday, he brushed aside a question about whether he was considering relinquishing the candidacy for chancellor to Soeder, saying that the Union had “decided clearly” who would run.