Impeachment doesn't necessarily have to involve a statutory crime that someone could otherwise be charged with. It's much more about good governance and making sure that elected officials are fulfilling their oath of office. If we viewed in that context, it's very clear that Trump isn't doing that.
That said, there's numerous crimes that Trump has, at least arguably, committed. One of these is extortion. Trump was holding up money which Congress it allocated to go to Ukraine in order to get something out personal political value. Another is a violation of the Hatch Act which Outlaws the use of government resources to promote political campaigns others others include failing to Faithfully execute the laws of the United States come in this case a congressionally mandated appropriation of money to Ukraine. Another might be potential violation of record laws, including the use of encrypted messaging apps for communication, and irony given their criticism of Hillary Clinton for her Communications. We now have potential obstruction of justice charges as the White House with holds testimony and documents from Congress.