WASHINGTON—Rep. Pete Stark was elevated Wednesday to acting chairman of the House Ways and Means Committee, but it remained unclear how long the sometimes-fiery California Democrat would hold on to the powerful post.

Mr. Stark, by virtue of seniority, became acting chairman after Rep. Charles Rangel, the New York Democrat, stepped aside to avoid facing a Republican-sponsored censure vote in the House amid a series of ethics inquiries. Few in Congress expect Mr. Rangel to retake the job.

If...