Over the weekend, President Donald Trump accused former President Barack Obama of wiretapping his phones during last year's presidential election. The president has provided no evidence for his claim, leading many to wonder, if the president's claims are baseless, did Trump defame his predecessor? 

While White House Press Secretary Sean Spicer called on Congress to investigate the allegations, fellow Republicans weren't ready to defend the president's claims, with some asking for the president to share evidence for his wiretapping claims with Congress and the American people. 
Obama himself has denied the charges, which seem to be based on a Breitbart news article. Legally, a president cannot order wiretapping of American citizens. But law enforcement can obtain warrants through the courts to implement wiretapping investigations.
If the claim does turn out to be baseless, then Trump could have libeled Obama, but only if Obama can prove it in court, something he is very unlikely to ever try and do. 
Libel is one of two forms of defamation, which is the act of publicly telling a false statement that harms the reputation of another person. Libel is committing defamation in writing, slander is the act of defaming someone through speech. 
In the U.S., defamation law can be traced back to the 1735 case of John Peter Zenger, a journalist in New York City who criticized the colonial governor. After Zenger was charged with libel, Zenger's lawyers successfully argued in court that because what Zenger had said was true, he should not be convicted of libel. Before Zenger's case, simply saying something that harmed another's reputation was enough to illicit a libel charge, regardless of whether the thing said was true. 
Zenger was criminally charged with libel. However, while there were a few states with criminal libel laws still on the books, convictions are extremely rare. Most defamation cases play out in civil court, with one person suing another.
The modern definition of defamation was established by the Supreme Court when it decided Sullivan v. New York Times in 1964. In that case, the court unanimously ruled that the L.B. Sullivan, the public safety commissioner in Montgomery, Alabama, could not sue the New York Times for libel, because although there were errors in an advertisement published in the Times about Montgomery's treatment of Martin Luther King, Jr., the Times did not show "actual malice" toward Sullivan. 
The ruling established that for public officials to win a defamation lawsuit, they had to show that the defendant not only lied about them, but did so on purpose, with the intent of hurting the official and his or her reputation.
A few years later, the court ruled that all public figures, not just public officials, had to show "actual malice" in order to win a defamation claim. 
President Trump has in the past expressed frustration with U.S. libel law, which give far more deference to free speech concerns than libel statutes in other countries. In February of 2016, the then-presidential candidate said he would "open up" U.S. libel laws to allow people to sue news publications. 
Whether or not Trump libeled Obama with his tweets would come down to whether or not what Trump said was false, and if it was, whether he made the false statement to intentionally harm Obama. 
Again, it is highly unlikely that Obama would ever sue the sitting president over a tweet. But if he were to file a lawsuit, he would have to also prove that the tweet was not an official "presidential act." 
In 1982, the Supreme Court decided that a president is immune from civil damages and civil liability while performing "presidential acts." So if a anyone were to file a defamation suit against Trump for a tweet sent from the White House, he or she would also have to prove that tweet wasn't sent as part of the Trump's official duties as president.