Trump's candidacy faces a 14th Amendment challenge.
GOP candidate John Castro alleges Trump's role in the Capitol riot disqualifies him
Legal experts believe this case could be the Supreme Court's toughest.
Little legal precedent makes the case unprecedented.
Decision on Trump's eligibility may spark protests.
Justices must decide who has standing to bring a 14th Amendment challenge.
Supreme Court may prefer to avoid ruling on Trump's eligibility.
Castro's case would focus on standing rather than Trump's candidacy.
Supreme Court tends to view such disputes as political questions.
Court might step in if a circuit split emerges or during primary states' ballot disputes.