WASHINGTON In a closely watched exchange during Wednesdays Senate impeachment trial, President Donald Trump s defense team struggled to point to any instances of his interest in rooting out corruption in Ukraine or Joe Biden s involvement with that nation before the former vice president launched his Democratic presidential campaign last April.

The key moment came in response to a question posed to Trumps lawyers by Sens. Susan Collins of Maine and Lisa Murkowski of Alaska, two Republicans who may prove crucial to whether the chamber agrees to hear from witnesses and allow the presentation of new evidence in the case. For that to happen, Democrats need at least four GOP senators to break with their leadership

The timeline of Trumps interest in Ukraine corruption is potentially crucial to the validity of the impeachment charges against him leveled by the House: that he withheld military aid to the country in exchange for its government opening an investigation into Biden and his son, Hunter, who served on the board of an energy company there while his father was vice president. If Trump and his team can establish that he cared about the issue before Biden began his campaign, or was gearing up for his run, it would undermine the charge that the president was acting merely in his own political interest his re-election.

But deputy White House counsel Patrick Philbin acknowledged he couldnt point to a specific example of an earlier interest in the corruption matter by Trump, arguing that the presidents defense was limited to addressing only evidence presented by the House impeachment managers.

Theres not something in the record on that. It wasnt thoroughly pursued in the record. So I cant point to something in the record that shows President Trump at an earlier time mentioning specifically something related to Joe or Hunter Biden, Philbin said.

Philbin did try to establish that Trumps interest in rooting out corruption in Ukraine predated Bidens presidential run because the presidents personal lawyer, Rudy Giuliani , expressed a desire to travel to the country a month before Bidens formally announced his presidential bid (speculation that he would run had been rampant for months).

Philbin also argued that one could draw a reasonable inference that Trump had simply gotten new information about the Bidens because of news coverage about his sons business ties in Ukraine prior to the presidents now-infamous call last July 25 with Ukranian President Volodymyr Zelensky. Trumps prodding of Zelensky to investigate the Bidens kicked the impeachment push into gear.

Rep. Adam Schiff (D-Calif.), the lead House impeachment manager, rebutted Philbin by noting the presidents lawyers could call a witness to offer evidence of Trumps interest in Ukraine prior to Bidens presidential run if they so chose.

Republicans are eyeing a quick acquittal vote after senators later this week consider the issue of whether to call witnesses and allow for new evidence.