A former top national security adviser to President Donald Trump has arrived on Capitol Hill to testify in the House impeachment inquiry.

Tim Morrison handled Russian and European affairs at the White House's National Security Council before resigning his position a day before Thursday's hearing.

Morrison, a defense hawk and former top Republican staff member on Capitol Hill, was aligned with Trump's former national security adviser John Bolton and raised concerns about the Trump administration's approach to Ukraine.

The impeachment inquiry is focused on Trump's effort to have Ukraine investigate Democrats and a potential 2020 rival, Joe Biden, while the administration was withholding military aid to the Eastern European ally.

It's illegal to seek or receive foreign help in U.S. elections. Trump says he did nothing wrong.