WASHINGTON, DC - FEBRUARY 06:  House Intelligence Committee Chairman Devin Nunes (R-CA) (L) and Rep. Peter King (R-NY) head for a House Republican conference meeting at the U.S. Capitol February 6, 2018 in Washington, DC. The committee voted unanimously Monday night to send a Democratic memo rebutting claims of bias in the Russia investigation FISA court application to the White House.  (Photo by Chip Somodevilla/Getty Images)
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WASHINGTON, DC - FEBRUARY 06: House Intelligence Committee Chairman Devin Nunes (R-CA) (L) and Rep. Peter King (R-NY) head for a House Republican conference meeting at the U.S. Capitol February 6, 2018 in Washington, DC. The committee voted unanimously Monday night to send a Democratic memo rebutting claims of bias in the Russia investigation FISA court application to the White House. (Photo by Chip Somodevilla/Getty Images)
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Editor’s Note: Julian Zelizer is a history and public affairs professor at Princeton University, editor of “The Presidency of Barack Obama: A First Historical Assessment” and co-host of the “Politics & Polls” podcast. Follow him on Twitter: @julianzelizer. The opinions expressed in this commentary are those of the author. View more opinion articles on CNN.

(CNN) —  

In a secretly recorded tape at a fund-raiser, California Republican Devin Nunes told the truth to a group of Republican donors. Not knowing he was being recorded, Rep. Nunes told the donors that the Republican majority in the House has been and remains the main firewall that has protected President Donald Trump from the fallout that could come from special counsel Robert Mueller’s investigation of whether there were links between the Russians and his campaign and of whether the White House has attempted to obstruct the investigation.

More than the President’s Twitter distractions and more than the conservative news media, it has been the Republicans in the House and Senate who have been vital to the survival of this presidency.

Speaking to the donors, Nunes said: “if Sessions won’t unrecuse and Mueller won’t clear the President, we’re the only ones. Which is really the danger … I mean we have to keep all those seats. We have to keep the majority. If we do not keep the majority, all of this goes away.” Nunes’ words made clear that the House’s investigation has been a charade designed to protect Trump rather than to reveal the truth.

In brazen fashion, Nunes can also be heard talking about the crass political calculations taking place – especially with regard to the Supreme Court nomination of Brett Kavanaugh and whether to attempt impeaching Deputy Attorney General Rod Rosenstein, who appointed and oversees special counsel Robert Mueller. The recording was aired by MSNBC.

Impeaching Rosenstein, an idea most politicians have agreed is purely a way to shut down Mueller, turns out to be an active project. It is just on hold because of timing, since Nunes recognizes that getting rid of Rosenstein could interfere with the Senate’s ability to confirm Kavanaugh to the Supreme Court. If Republicans keep control of Congress, the move against Rosenstein – and Mueller’s investigation of Trump – might be coming next.

This is another indication of why the midterm elections will be so important to the next two years of the Trump presidency. This is why Trump has been so active on the campaign trail, although there is growing evidence after Tuesday that his presence at rallies around the country might do more than anything else to hand Democrats both chambers.

As chair of the House Intelligence Committee, Nunes has fought tooth and nail against allowing the lower chamber to conduct a full-throated Watergate-style investigation into what kind of interference occurred in the 2016 election. Besides limiting the scope of the inquiry and shutting down hearings very early on, Nunes also led Republicans in the effort to fight the legitimacy of the investigation itself. It was the infamous Nunes memo that spelled out the allegations of a “Deep State” conspiracy by the FBI to undermine the chances of Trump’s victory against Hillary Clinton.

Most important, Nunes has not acted alone. He worked under the auspices of the leaders of the Republican Freedom Caucus, North Carolina’s Mark Meadows and Ohio’s Jim Jordan, to do everything possible to constrain the scope of the investigation into Trump and to redirect the energy toward questioning the motives and behavior of the investigators themselves. Most Senate Republicans have been equally quiescent, doing very little as they observe the President’s constant barrage against the “witch hunt” and outlandish moments such as deciding to go after US intelligence agencies while standing next to Russian leader Vladimir Putin.

Nunes’ comments appear to be an abdication of the constitutional role of Congress, to act as an independent branch of government that conducts oversight and serves as a check and balance. The comments, as well as the stance of a majority of Republicans, stand in stark contrast to the way that North Carolina’s Sen. Sam Ervin pursued his congressional investigation during the Watergate scandal.

Nunes and other Republicans have been crucial to helping the President in other ways as well. They have stood by quietly in light of ongoing evidence of corruption and conflict of interest at the top of several executive departments. The contrast with the number of investigations that the Republicans conducted into the Clinton administration, for allegations much more minor than the ones Trump has faced, is revealing.

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Even with all the stories about ousted Cabinet officials like Tom Price and Scott Pruitt, congressional Republicans have generally dropped the ball on much of their oversight functions. In an administration which many ethics experts agree has thrown the restrictions on conflict of interest out the window – as the Trump presidency and Trump business sometimes feel like they are merging into one – House and Senate Republicans have sat on their hands, watching their television sets and Twitter feed with a cringe, as the swamp keeps getting deeper.

Republicans backed the President during two pivotal moments in his term – the confirmation of Neil Gorsuch to the Supreme Court and the passage of the corporate tax cut, through which the President delivered major victories to core Republican supporters. Those congressional victories have been extremely important in explaining why the President’s support remains much stronger within the GOP despite his incredibly provocative and outlandish behavior. These are the victories that the President counts on to bolster Republican turnout in the midterms.

Yes, in private, Congressman Nunes offered some of the most honest words thus far from congressional Republicans on the Hill.

He also made the case better than most as to what is at stake this November. Although the midterms are elections about the composition of Congress, they will do more than anything else to dictate the future of the Trump presidency.