Former White House official Fiona Hill told Thursday's impeachment hearing that a "fictional narrative" about Ukraine, driven by partisan politics, distracted President Trump from the real threat that Russia poses to America's democracy.

Why she matters: Hill, who left last summer as Trump's top adviser on Russia and Europe, gives House investigators a window into former national security adviser John Bolton's objections to Trump's Ukraine activities. In closed testimony last month, Hill said Bolton called Rudy Giuliani a "hand grenade that is going to blow everybody up."

In a brief opening statement, Hill emphasized that she believes it is her patriotic duty to answer the House Intelligence Committee's questions about what she saw and what she knows, according to a source familiar with her testimony.

Hill spoke about where she comes from: She is descended from generations of coal miners in the U.K., and her family’s love and respect for America is why she became an American citizen.

Hill, who has served under three different Republican and Democratic presidents, addressed why she has dedicated her career to being a nonpartisan foreign policy expert.

Between the lines: Republican aides working on impeachment have conceded to Axios that Hill was a damaging witness for Trump during her closed-door testimony.

While EU Ambassador Gordon Sondland had spoken directly with Trump about the investigations, Hill is the witness with the strongest ties to the West Wing.

She will shed light on key conversations with Bolton, and had a front seat to the politics of diplomacy with Ukraine.

And, as Trump’s top Russia adviser, she will likely drive home the importance of the aid for Ukraine, and how any crack in the U.S.-Ukraine relationship could be seen as a bolstering Russia.

Read Hill's opening statement.

Go deeper: The schedule for this week's impeachment hearings