> Could the constitution not be amended, at least in theory?

Sure, but amending the Constitution for the purpose of permitting the government to impose greater limits on political speech is...going to be a hard sell for a lot of people, and have a high risk if successful of having pretty severe unintended (for non-autocratic supporters of whatever the particular measure is) consequences.

> Or the supreme court create a new interpretation?

Easier than passing an amendment, similar downsides.

Not in our generation. The current thought process of conservative jurists is that money is speech and the founders wanted corporate entities to have unencumbered speech.