The decision to subpoena former special counsel Robert Mueller to testify before the House Judiciary and Intelligence committees this week represents a Hail Mary pass for congressional Democrats. It comes after earlier congressional efforts to publicize his findings have fizzled and polls show that public support for impeachment may be waning.

Democrats seek riveting sound bites from Mueller chronicling President Donald Trump’s misdeeds. But their strategy has a significant risk that should not be underestimated. The questioning by Democrats — and more importantly by Republican members — may create a spectacle that could undermine Mueller’s credibility with the public and, therefore, his report.

The reason House Democrats have subpoenaed Mueller is apparent. Few Americans have read all of Mueller’s 448-page report detailing his findings about Russian interference in the 2016 election and the 10 alleged episodes of obstruction of justice by Trump.

And Democrats’ other efforts to draw widespread attention to the most damning parts of the report haven’t worked as well as they had hoped. On May 19, Democratic House members read excerpts from Mueller’s report; the public did not really notice. On June 10, the House Judiciary Committee questioned Nixon-era White House counsel John Dean and knowledgeable legal experts about Mueller’s report; the impact of that hearing was blunted by breaking news about a fatal helicopter crash in midtown Manhattan.

Thus, the thinking goes, if the public watches Mueller highlight his evidence on television, and if the hearings generate compelling and readily-understood visual confirmation of the evidence, Democrats could use his testimony to sway public opinion, if not in favor of impeachment, then in political advertisements in 2020.

That Mueller can shape public perceptions is well documented: After Mueller’s brief public press statement on May 29 about his report, several Democratic presidential candidates immediately endorsed impeachment efforts.

Mueller’s impact has been seen in a more prosaic way, too. In Ireland and England, where bookmakers allow gambling on American politics, the Irish bookmaker BoyleSports dropped the odds that Trump will be impeached by the House from 8-1 to 2-1 immediately after that public Mueller statement. Little wonder the Democrats want Mueller to testify on live television.

However, Mueller’s stated position that he will not testify to anything beyond what is contained in his written report injects serious risk to the success of the Democrats’ strategy. "There has been discussion about an appearance before Congress. Any testimony from this office would not go beyond our report," Mueller said on May 29.

Known as a man of his word, Mueller is unlikely to budge from that limitation.

Democrats on the Judiciary and Intelligence committees may — and, in fact, almost certainly would — nonetheless ask Mueller open-ended questions in the hopes that the normally taciturn witness would give quoteworthy narrative answers laying out a case against Trump for obstruction of justice. That outcome is unlikely.

If Mueller won’t give narrative responses summarizing his report, House Democrats could read excerpts from his report and ask if those statements are true. Mueller would likely answer in one word: Yes. Such repeatedly terse answers would utterly fail to give the TV-worthy impact that the Democrats seek and would not likely move the needle of public opinion.

Still, other than the hearings being a wasted opportunity, the Democrats would not be much worse off for having subpoenaed Mueller.

But it could go so much worse than that. Unless the Democratic Congress members are highly disciplined, they may ask Mueller to reveal information not in his report. When Mueller refuses to do so, whether by citing grand jury secrecy or simply an unwillingness to elaborate, the American public will observe a witness who appears less than forthcoming.

Trial lawyers understand that the demeanor of a witness is as critical as his testimony. If Mueller deflects some questions or simply refuses to answer them, the visual impact may lead the public to doubt the veracity of what he does answer.

But the biggest problem for Democrats will be the questioning by Republican committee members. They are likely to seize this opportunity to question Mueller about the genesis of the investigation of Trump’s campaign or the Obama administration’s application for Foreign Intelligence Surveillance Act (FISA) warrants to authorize wiretaps on Carter Page, a Trump campaign adviser.

Republicans will probably also ask Mueller about Attorney General William Barr’s characterization in congressional testimony that “spying did occur” against Trump’s campaign, referring to the origins of the Russia investigation. If Mueller refuses to answer, Republicans may then also assert that Mueller did not disagree with Barr’s claim that the investigation began with the FBI spying on the president.

This week will reveal whether compelling Mueller to testify publicly was a brilliant strategy or a failure. And the prospects of initiating an actual impeachment investigation may depend upon which it turns out to be.