It is an obvious line of attack that has been creeping into the arsenal of rivals trying to stop former President Donald J. Trump ahead of the Iowa caucuses on Monday — if nominated to be the Republican Party’s White House standard-bearer, the former president could very well be a convicted criminal by Election Day.

Gov. Ron DeSantis of Florida inched toward that cudgel at a debate on Wednesday night, warning that a “stacked left-wing D.C. jury” is likely to sit in judgment of Mr. Trump’s efforts to subvert the 2020 election, and asking, “What are the odds that he’s going to get through that?”

Then, he added, “what are we going to do as Republicans in terms of who we nominate for president? If Trump is the nominee, it’s going to be about Jan. 6, legal issues, criminal trials.”

Former Gov. Nikki Haley of South Carolina has been far more reluctant to broach his legal troubles, speaking almost daily of Mr. Trump as an agent of “chaos” and “disarray” without explicitly mentioning the 91 felony counts looming against him.

But perhaps taking their cues from voters leery of attacks on the former president, Mr. Trump’s closest rivals continue to avoid one ominous word: conviction.