Questions swirling about Russian election meddling and the Trump campaign's ties to Vladimir Putin have spurred wild conspiracy theories from Trump opponents and dismissals from the president and his supporters that this is a "witch hunt."

The guilty plea on Friday of former national security adviser Michael Flynn should serve as a beachhead to those sorting through this sea of misinformation. In a stunning turn of events, Flynn has acknowledged that he lied to the FBI while in office about conversations he'd had with the Russian ambassador as part of Trump's transition team in late 2016.

That this former high-ranking White House official and prominent campaign adviser has admitted to a felony doesn't prove that President Donald Trump did anything wrong or, more specifically, that he conspired with the Russians during the campaign — a key focus of special counsel Robert Mueller's investigation. But Flynn's plea is an important step forward in replacing Russia spin with fact.

It should also make it difficult, even for Trump supporters, to argue that further scrutiny is not needed. There does appear to be substance to concerns about the campaign's Russian contacts and the consequences this might have had. Getting to the bottom of this is in the president's best interest and that of the nation.

Questions about Trump's ties to Russia aren't going away, and they're casting a pall over his presidency. Understanding how a foreign power may have tried to interfere with the 2016 election is also critical to protect future elections' integrity. That Mueller is on the job is reassuring, especially when other efforts to investigate or put safeguards in place have made little progress.

There is clearly much more work to be done, which is why Mueller and his team must be allowed to finish the investigation, wherever it may lead. That's why it is disappointing that Congress has failed to enact safeguards to protect Mueller, especially after some reports indicate that Trump has considered measures to fire him.

Rather than wrapping up soon, as the president's lawyer Ty Cobb has suggested, the Flynn plea suggests this probe is gathering momentum. It's widely expected that Flynn's cooperation has or will yield further details about the president and his inner circle. The information may also shed unflattering light on Trump's curious persistence earlier this year in trying to shield Flynn from investigation. Trump may well conclude that terminating Mueller would be a quick solution.

Congressional leaders must make it immediately clear that firing Mueller would have serious consequences — ones that would rapidly escalate any scrutiny the president seeks to avoid. Right now, U.S. House and Senate leaders are focused on tax reform. But their voices may be the only ones that Trump heeds at this critical juncture.

And more than most, congressional leaders are keenly aware of Trump's discomfort with questions about Russian election meddling. This week, the New York Times reported that Trump urged top Republican senators to swiftly end an Intelligence Committee investigation into this matter.

The senators thankfully didn't heed this request. But they may soon need to do more than say no to the president. If this happens, the world will be watching to see if they champion accountability or take the easy way out.