WASHINGTON — The last time he was accused of collaborating with a foreign power to influence an election, he denied it and traveled the country practically chanting, “No collusion!” This time, he is saying, in effect, so what if I did?

Even for a leader who has audaciously disregarded many of the boundaries that restrained his predecessors, President Trump’s appeal to a foreign power for dirt on former Vice President Joseph R. Biden Jr. is an astonishing breach of the norms governing the American presidency.

That his phone call with Ukraine’s leader took place literally the day after the special counsel Robert S. Mueller III testified to Congress about Russian interference in the 2016 election demonstrated that Mr. Trump took no lessons from that episode about the perils and propriety of mixing his own political interests with international relations.

If anything, the president has grown even more defiant since Mr. Mueller found insufficient evidence to establish a criminal conspiracy between the Trump campaign and Russia, almost as if having avoided charges, he is daring the establishment to come after him again. The man who once said he could shoot someone on Fifth Avenue in Manhattan without consequence seems to be testing whether he can do the political equivalent.