Democrats in the House of Representatives want President Trump's tax returns. The House Ways and Means Committee yesterday asked for Trump's returns from 2013 to 2018. The only way would mean breaking the law.
Tax returns are private. You can ask, but the politician can say no.
For example, Nancy Pelosi is in the line of presidential succession. You can ask to see the return she and her husband filed. She says no. End of story.