The Mueller investigation has started to interview (under oath) some of the people highest up in the Trump administration. They just finished interviewing Attorney General Sessions, and will be interviewing Steve Bannon next week. That means they are getting close to wanting an interview with Donald Trump.
For his part, Trump has backed off his earlier claim that he would be happy to testify under oath. He is now claiming that it's not necessary for him to testify (since he claims no collusion has been proven). That's not going to fly though. His lawyers know that, and they are doing their best to either limit his testimony, or have it done in written form. That is silly. Clinton testified under oath when being investigated, and you can bet that Mueller will want Trump to do the same.
That's also what the American people want. They say, that if asked by Mueller to testify, Trump should do it in person and under oath. About 78% want that, while only 18% don't -- an enormous gap of 60 points.Trump is already the most unpopular president since World War II, and trying to get out of testifying is not going to help him.
The chart above is from a new CNN / SSRS Poll -- done between January 14th and 18th of a random national sample of 1,005 adults, with a margin of error of 3.7 points.