Julian Zelizer is a history and public affairs professor at Princeton University and the author of " The Fierce Urgency of Now: Lyndon Johnson, Congress, and the Battle for the Great Society. " He's also the co-host of the " Politics & Polls " podcast. The opinions expressed in this commentary are his own.

(CNN) Special Counsel Robert Mueller's investigation has come under fierce political attack. President Donald Trump and his allies are systematically attempting to destroy the legitimacy of the investigation in the eyes of the public. And without a strong congressional investigative counterpart, Mueller finds himself increasingly isolated and alone.

While the White House issued a recent statement that it has no intention of firing Mueller, that is almost beside the point. In what should now be considered the classic Trumpian playbook, the President has moved aggressively to raise doubts about the credibility of his opponent. Ironically, he and his allies are attempting to crush an investigation into whether his campaign colluded with the Russians by insinuating that the Hillary Clinton campaign may, in fact, be at fault for such behavior.

The President's attacks should not be taken lightly. As Brian Stelter has argued on CNN, Trump and the conservative media have perfected echo chamber politics, whereby the multiple charges about the investigation -- that FBI agents were out to systematically bring down this presidency, that the agency is damaged by rampant conflict of interest problems, that Mueller is illegally obtaining information about the transition -- have moved to the forefront of the national conversation regardless of the veracity or relevance of any of these claims.

Peter Carr, a Mueller spokesman, made a statement soon after the allegation emerged: "When we have obtained emails in the course of our ongoing criminal investigation, we have secured either the account owner's consent or appropriate criminal process."

The stories bleed into the rest of the media as well. On Sunday morning, a Washington Post headline read, "Mueller unlawfully obtained emails, Trump transition team claims," which was likely music to the President's ears. An allegation by the Trump for America legal team had quickly made its way into the headlines.

Indeed, it is telling of how effective Trump can be that Mueller's decision to fire an FBI agent for his email conversations about the campaign was somehow turned into a black mark against him, rather than a sign of how cautiously the process has been handled.

Mueller is hamstrung in his ability to respond. If he is too aggressive in attempting to fight back against these political attacks, he opens himself up to charges of being "too political." If he is too quiet, Trump's team will destroy his credibility by Christmas.

In contrast, Trump and his team can basically throw the kitchen sink at him without any restraint. Trump has Twitter, the conservative media, his congressional allies and an army of political surrogates who all will help him in this goal.

The current onslaught against Mueller is a powerful reminder of why the absence of a vigorous congressional investigation to complement Mueller's work is so damaging. This has also been the biggest difference between now and the 1970s Watergate investigation that so often evokes comparisons to our current times.

When prosecutors Archibald Cox and Leon Jaworski conducted their investigations into Richard Nixon, they were not standing alone. The Democratic Congress conducted its own hearings, which were aggressive, public and robust -- and allowed the public to see firsthand what had taken place in Nixon's administration.

Those congressional hearings helped build public support for the case against Nixon and created a firewall against his efforts to stop the investigation. There was an entire, co-equal branch of government that would not easily succumb to attacks from the person being investigated. In that case, divided government proved to be essential to the abuse of power.

None of that is true today. We have a highly partisan Republican Congress in charge of investigating a Republican President. Added to that fact is the massive conservative media universe that broadcasts Trump's point of view.

Mueller, who is a shrewd prosecutor and familiar with the ways of Washington, is working in relative isolation. As he faces this current challenge, with a meeting scheduled with White House counsel this week, he has few allies in Washington other than powerless congressional Democrats who can't do much to stand up for him.

Thus far, the Republicans in Congress have proven to be a feckless bunch when it comes to standing up to the President's most outrageous actions. Indeed, right now congressional Republicans such as Ohio's Jim Jordan seem to be more concerned about investigating Mueller than the possibility that Trump's campaign colluded with the Russians and that the President may have obstructed the investigation.

Mueller might very well be counting on something that just doesn't exist: political outrage from the Republican Congress. It could be that the President and his team tar and feather the special counsel. They may even go as far as they did with former FBI Director James Comey by firing him, and that life on Capitol Hill simply goes on.

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Sure, a few Republicans like Senators Bob Corker of Tennessee and Jeff Flake of Arizona might issue some angry statements, but in the end, they will likely do nothing.

Mueller's best ally has been himself and the hard work of the counsel's office. The indictments thus far have been powerful and, at least until now, the public has been supportive of what he is doing.

But there are limits to what Mueller can do on his own, especially as the President and his allies intensify the barrage of attack. This can quickly extract its toll on the reputation of the investigation. Thus far, despite Mueller's public support and the President's strong disapproval ratings, it is also important to note that Congress has been quite restrained in how it has handled this and other damaging issues related to the Trump administration.

Meanwhile, the public has heard very little from the congressional committees investigating the matter, and there has been substantial evidence that partisanship has stifled much of the serious work that they have to do. In fact, in October, the New York Times reported that the House and two Senate committee investigations have encountered serious problems -- from inadequate staffing to delays and partisan disputes over the scope of the inquiry -- that have hindered their work.

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The coming weeks will be crucial to the longevity of this investigation. How far will the President go to bring this to an end, and what happens if he takes the most draconian steps possible, such as firing Mueller and closing his investigation?

The main issue will be whether Trump's actions even matter anymore or whether we live in a partisan world where there are no boundaries for a president who governs in a united government.

The only legitimate way this investigation should end is by Mueller saying it is over. But if this story ends a different way, one that some would consider to be a constitutional crisis, will Mueller be like that tree that falls in the forest when no one is around to hear the sound?