And you need probable cause to obtain such a warrant.
So if indeed Trump and his associates' calls were monitored, that means that a court found probable cause.
Trump opened a can of worms with his stupid Tweet.
If there was no warrant issued, then he's a liar.
If there was, then a court found that there was enough probable cause to issue such a warrant, which doesn't make him look very good.
Once again he stepped on his own dick with one of his crazy early morning Twitter rants.
The guy can't seem to stay out of his own way.