Was Meriwether Lewis a Democrat or Republican?

Answer 1

The modern Democratic Party wasn't formed until 1828, or 19 years after Meriwether's death by suicide. However, that party had historic roots back to 1792.

While the main parties then were "Democratic-Republican" and "Federalist", President Thomas Jefferson was of the "Democratic-Republican" party.

The modern Democratic Party came by a split of the "Democratic-Republican" party, and those who called themselves the Democrats were adhereing to Jeffersonian principles.

All that is important as Meriwether Lewis was a political appointee of President Jefferson, so one may infer he was a Democrat.

(However, the party's ideology has changed substantially in the two centuries since Meriwether's death.)