BERLIN — Chancellor Angela Merkel has reigned as Germany’s — and often Europe’s — leading political power for 13 years. But within hours of her announcement on Monday that she would step down this year as leader of her conservative party, focus had shifted to who could possibly replace her.

Ms. Merkel said she would not run again when the Christian Democrats elect a leader in December, relinquishing a post she has held since 2000. She said she planned to serve out her fourth term as chancellor, through 2021, but in light of strains within her party and its coalition partners, even many people within the party think she is unlikely to remain in power until then.

Whoever is elected to lead the Christian Democrats after Ms. Merkel would be well positioned to eventually succeed her as Germany’s leader, as well, greatly raising the stakes in the December contest. As the power struggle shapes up, here are some of the potential candidates: