This can be read in the 6-page letter by Trump to Congress about the articles of impeachment:
The second claim, so-called "Obstruction of Congress," is preposterous and dangerous. House Democrats are trying to impeach the duly elected President of the United States for asserting Constitutionally based privileges that have been asserted on a bipartisan basis by administrations of both political parties throughout our Nation's history.
Under that standard, every American president would have been impeached many times over. As liberal law professor Jonathan Turley warned when addressing Congressional Democrats: "I can't emphasize this enough...if you impeach a president, if you make a high crime and misdemeanor out of going to the courts, it is an abuse of power. It's your abuse of power. You're doing precisely what you're criticizing the President for doing."
(Any typos are mine - I couldn't find a text version)
@DavidSchwartz summarized it very well in a comment (quoted with permission):
The issue is to what extent the President, representing a branch that supposed to be equal to Congress, has to cooperate in an attempt to impeach him. Trump's argument is that he contested the subpoenas in court because he believed they were illegitimate and it is not an abuse of power to ask courts to decide a contested legal issue where he has arguably asserted a recognized privilege. Congress' argument is that they get to decide what the President is legally required to do because the remedy for such abuses would be impeachment and that's in their sole discretion.