President Trump's lawyer Rudy Giuliani told Fox Business' Maria Bartiomo Monday that he couldn't assure her "100%" that Trump had not threatened to withhold military aid from Ukraine if the country did not launch an investigation into Joe Biden and his son.

The big picture: Giuliani is at the heart of the Ukraine controversy — which he says involved him asking Ukrainian officials to investigate "Ukrainian collusion" that he claims occurred during the 2016 presidential election — but he acknowledged over the weekend that his request involved Biden.

Watch the exchange:

The big picture, via Axios' Jacob Knutson: House Democrats are now investigating Giuliani's time in Ukraine and whether Trump pressured Ukraine's president to open an investigation into Biden.

Trump confirmed on Sunday that he discussed Biden with Ukrainian President Volodymyr Zelensky during a July phone call.

Go deeper: What you need to know about the Trump-Ukraine controversy