How do you handle disagreement with senior stakeholders about marketing direction?

Show that you advocate with evidence and then commit.

• Understand what is actually driving the position. Often the disagreement is about an underlying concern — cash flow, a board expectation, a competitor's move — rather than the tactic being discussed. Addressing the real concern resolves more disagreements than arguing the tactic.
• Bring evidence, not conviction. Historical performance, a test result, competitor analysis, or customer research. "I think" loses to "here is what happened when we tried it".
• Propose a test rather than demanding a decision. Running a limited version resolves the argument with data and lets both sides move without either being wrong. This is the single most useful technique.
• Be clear about the risk you see, stated once and plainly, so the decision is made with full information.
• Then commit fully. If overruled, execute properly rather than half-heartedly and then pointing at the result. Undermining a decision you lost is what ends careers.

Note: Describing a time you were overruled, executed well, and were proved wrong is a strong answer — it shows you can hold a view without being certain you are right.