From The National:

Paul Rosenzweig, a legal expert at R Street institute and a former deputy assistant secretary for policy at the Department of Homeland Security, said there was a clear case for impeachment.

“President Trump is essentially daring the democrats to impeach him. If they fail [to impeach him in the House], he will be able to establish a presidential norm that it is acceptable to use his powers to solicit information from a foreign government,” Mr Rosenzweig told The National.

But unlike the Mueller probe, which mostly involved Mr Trump’s campaign aides and not the president directly, Mr Rosenzweig said the impeachment case “is very clear” in that the US president is promising the Ukrainian leader “money for dirt on my opponent”.

The money in this case is US military equipment, but Mr Trump denies any “quid pro quo”.

“He is not just seeking assistance from a foreign government, but he is also giving them leverage in US politics,” Mr Rosenzweig argued.

“Unlike [former President] Richard Nixon who kept interference to a small circle of aides around him, Mr Trump has involved a large number whose names we see getting dragged into the investigation,” Mr Rosenzweig argued.

Mr Rosenzweig expected other Senators such as Susan Collins or Cory Gardner, who are up for re-election, to vote against Mr Trump, but still without reaching 67, the magic number required to convict the president.