Given the avalanche of new details in the Trump-Russia probe and whether President Trump obstructed justice in firing FBI Director James B. Comey, it’s not surprising that Republicans are fortifying the president’s defenses. On Thursday, Sens. Charles E. Grassley (R-Iowa) and Lindsey O. Graham (R-S.C.) released a letter in which they claim Comey “prejudged” the investigation into Hillary Clinton’s email practices. Trump latched onto the story, tweeting Friday morning, “Wow, looks like James Comey exonerated Hillary Clinton long before the investigation was over…and so much more. A rigged system!” Rather than undermine the case against firing Comey, though, Grassley and Graham’s letter and the reactions by other Republicans show again that they’ll use any excuse to hurt Comey’s credibility and by extension special counsel Robert S. Mueller III’s inquiry.

Grassley and Graham’s letter centers on partial transcripts of interviews the Office of Special Counsel conducted with FBI officials last fall. These officials said that Comey began to work on a statement exonerating Clinton as early as April 2016, before Clinton herself and some key aides were interviewed. “Conclusion first, fact-gathering second — that’s no way to run an investigation,” charge the two Republicans. (Let’s pause for a moment on the irony of two members of the “Lock Her Up” party accusing someone else of jumping to conclusions.)

But it’s far from unusual for prosecutors and investigators to draft statements about their conclusions before the investigation is over so that the case can proceed efficiently. If Comey and his team had taken weeks after interviews were finished to draft and announce his decision, Republicans would have claimed he was trying to slow-walk the case. That Comey was already at work on a statement is not evidence of impropriety. As Washington veterans (and in Graham’s case, a former lawyer), the senators likely know this.

The letter’s very first sentence exposes the phoniness of Grassley and Graham’s outrage. It puts the letter squarely in the context of “the circumstances surrounding Director Comey’s removal, including his conduct in handling the Clinton and Russia investigations.” But the “circumstances” the Trump White House cited in removing Comey never included anything about Comey going easy on Clinton. The White House first relied on a memo from Deputy Attorney General Rod J. Rosenstein that criticized Comey not for going easy on Clinton, but for releasing “derogatory information” about her in his July 2016 statement and reopening the case in October (a decision Republicans praised at the time). Other excuses the White House offered included Comey making factual errors in testimony to Congress and Comey’s failure to prevent leaks to the press. But the factual errors made Clinton and her aides look worse, and the leaks often hurt Clinton as well.

Besides, as the president admitted and as Grassley and Graham certainly know, Comey was not fired because of his treatment of Clinton. “When I decided to just do it,” Trump told Lester Holt two days after Comey’s firing, “I said to myself — I said, you know, this Russia thing with Trump and Russia is a made-up story.” Further evidence that the firing was about the Russia investigation came Friday: News broke that Trump and top aide Stephen Miller drafted a multi-page letter to explain Comey’s dismissal, which included “Trump’s frustration that Comey was unwilling to say publicly that Trump was not personally under investigation” in the Russia probe.

To be clear, Comey fumbled badly in his handling of the Clinton email probe, particularly in his decision to reopen it in the election’s waning days. But Grassley and Graham know that Trump didn’t remove him because of the Clinton investigation. The only reason for the pair to dredge this up is to undermine Comey’s credibility, grab some favorable headlines in conservative media and hurt Mueller’s investigation. It’s a cheap stunt that both senators should be ashamed of.