The decision by House Speaker Nancy Pelosi (D-Calif.) to bar two of Minority Leader Kevin McCarthy’s (R-Calif.) choices to serve on the House select committee investigating the Jan. 6 Capitol riot is, to steal a phrase, worse than a crime. It was a blunder.

Let’s grant for the sake of argument that the two people she vetoed, Reps. Jim Banks (R-Ind.) and Jim Jordan (R-Ohio), were predisposed to whitewash President Donald Trump’s involvement in the riot and its aftermath. Let’s also grant, hypothetically, that they were inclined to try to pin blame on the speaker or her allies with respect to the steps that Capitol Police had or had not been directed to take on that fateful day. Neither of those predispositions, if true, would disqualify them from serving on the committee.

The committee needs to do more than simply gratify Democratic desires to unearth factoids that they can spin into blaming Trump for trying to, at worst, stage a coup. It needs to investigate energetically all potential causes, primary and secondary, for what happened. It also needs to allow people who strongly disagree with its initial premise to participate precisely so that they have the opportunity to throw the investigation off its course. Only by doing both of those things will the committee have a strong case that will withstand the inevitable partisan assaults from Trump’s die-hard defenders.

Republican consent to the committee’s findings is not necessary for them to be seen as credible, but Republican participation is. Imagine if the Watergate committees tasked with investigating President Richard M. Nixon’s crimes had excluded the Republican members who were defending the president. Those committees changed public opinion because those defenders, despite their best efforts, could not credibly argue against what the committee unearthed. Nixon began his second term in January 1973 with a 68 percent Gallup approval rating, but by August 1974, 57 percent of Americans believed he should be removed from office. That could not have happened without the active participation of members who were viscerally opposed to the investigation from the beginning.

Without Republican appointees being able to freely participate in the Jan. 6 committee, we are almost guaranteed that we will not find out the full truth. It’s entirely possible, for example, that Trump’s pre-riot actions stoked the flames that led to the riot and that the Capitol Police’s ill-preparedness is what let the rioters into the Capitol itself. Would that day have turned out differently if the rioters had been held at bay, even if the confrontation between police and protesters had turned violent? It’s in the public’s interest to find out. But now, it’s reasonable to expect that the remaining people on the committee will look primarily, if not exclusively, into Trump’s actions, as is their stated intent. The result will be seen for what it is: an attempt to score political points rather than an attempt to discover the truth.

The sad fact is that opposition to Trump is nearly the only thing that unites the Democratic voter coalition. Forget disagreements between Democratic progressives and centrists, as important as they are. The coalition that elected Biden contains millions of people who aren’t Democrats — the independents and Republicans who just couldn’t bear another four years of Trump. Some of them flipped back to vote Republican for Congress and state offices in 2020, and more can be expected to do so as older, traditional political divisions reassert themselves in the coming election cycles. Keeping the focus on Trump and his odious misdeeds, then, is in the Democrats’ narrow partisan interests. It’s the 21st-century version of the GOP’s tactic in the post-Civil War era to “wave the bloody shirt,” the constant argument that the Democrats — then primarily elected by white Southerners — were rebels and traitors.

That argument only flies if the charges ring true, which is why Pelosi’s decision is a mistake. Without GOP involvement in the process, only partisans will see the findings as convincing. That’s what happened during the first impeachment investigation: Trump’s job approval rose the longer House Intelligence Committee Chairman Adam B. Schiff’s (D-Calif.) nakedly partisan investigation continued. People can see when they are being conned.

Trump may be strengthened if the committee’s zeal leads it to trot out findings that are subsequently debunked by a more through and less partisan investigation. Failure to extensively probe not just the riot’s cause but also how it breached the Capitol will leave Republicans room to conduct their own special investigation into the Capitol Police in 2023 if they regain control of a congressional chamber. A partisan Jan. 6 committee won’t end the dispute over what happened that day; it will simply ignite and extend it.

That might be in the Democratic Party’s interest, given the difficulties it has uniting its fractious elements. But then Pelosi’s decision is a case of putting party over country. That’s a blunder that will ultimately hurt all of us.