He attempted to soften his immigration stance a while back. He tested the water with Sean Hannity and then did a 180. Also some of his supporters expressed betrayal when Trump bent to pressure to endorse Paul Ryan in his primary run. Basically his strength is his weakness, so anything along the lines of "you know what, I think amnesty for illegals is a good idea" would sink his campaign. In addition, there are hard-core conservatives who dislike Trump but will vote for him because they think it would be better for conservatives in the long run. If Trump dispelled this notion with "I would support the nomination of Merrick Garland or [insert left-leaning justice here]" or "I think we should raise taxes to balance the budget" then he would lose a large chunk of his support. Democrats would do well to press the first issue. They should run ads showing Trump's inconsistency on immigration by replaying bits of the Sean Hannity interview and comparing them to harsher things he's said on the issue and caption it with "Which Trump do you think would show up to the Oval Office?"