On the day a key former FBI official is set to testify on Capitol Hill, President Trump’s months-long crusade to paint the special counsel investigation into Russian interference in the 2016 election as a partisan “witch hunt” appears to have turned the question into another 50-50 partisan issue, according to a new Washington Post-Schar School poll.

Americans are nearly split, 49 percent approving and 45 percent disapproving of the way special counsel Robert S. Mueller III is handling the investigation into possible ties between Trump’s presidential campaign and the Russian government.

What’s more, 48 percent say the question of whether Trump’s campaign colluded with Russian efforts to influence the 2016 election is a “serious issue,” while an identical 48 percent say it’s “more of a distraction.”

On Thursday, the FBI faces another round of scrutiny, as former top FBI counterterrorism official Peter Strzok testifies before Congress, putting a focus on potential anti-Trump partisanship in the special counsel’s investigation.

Strzok has become central to allegations of political bias in the probe after the revelation he sent text messages that disparaged Trump during his presidential campaign to Lisa Page, a former FBI lawyer with whom he was having an affair, including one in which Strzok said that “we’ll stop” Trump from making it to the White House. Strzok was removed from the special counsel investigation last month.

At the beginning of 2018, Mueller held a healthy reservoir of public support, with a national Washington Post-ABC News poll finding 50 percent approved of his handling of the investigation, far higher than the 31 percent who disapproved. And that was down from November, when 58 percent approved of Mueller. Those surveys conducted by cellular and landline phones are imperfect comparisons to the current 49 percent approval-45 percent disapproval margin, which was conducted largely online via NORC’s probability-based survey panel. Nonetheless, Republicans appear to have moved sharply in line with Trump’s rhetoric.

In the new survey, 77 percent of Republicans and Republican-leaning independents disapprove of Mueller’s performance while 22 percent approve. That’s a far more lopsided margin than in the January telephone poll in which 50 percent of Republican leaners disapproved. Democrats and Democratic-leaning independents are a mirror opposite, with 76 percent approving of Mueller in the new survey conducted mostly online, slightly higher than 68 percent in the January phone survey.

Similar to the new Post-Schar School poll, a June CNN poll conducted by SSRS found a comparable share of Americans approving (41 percent) and disapproving (39 percent) of Mueller’s handling of the investigation. Other recent polls have found the special counsel maintaining majority confidence, including a June Pew Research Center survey finding 58 percent “very” or “somewhat” confident that Mueller will conduct a fair investigation, while 40 percent were “not too” or “not at all” confident. Even then, the Pew survey found that Republican skepticism was growing.

The Post-Schar School poll included an oversample of Americans living in congressional districts competitive in November’s midterm elections, and found slightly fewer Democrats and Democratic-leaning independents in battlegrounds (71 percent) who approve of the investigation compared with 77 percent in non-battlegrounds. There’s little difference among Republicans and Republican-leaning independents who approve of Mueller’s investigation in battlegrounds (21 percent) compared with non-battlegrounds (22 percent).

Among independents who lean toward neither party in battleground districts, 55 percent approve of the Mueller investigation while 42 percent disapprove.

Beyond Mueller, the partisan chasm is even wider over the importance of finding out whether Trump’s campaign colluded with Russian efforts to influence the election, which Trump has vehemently denied. Almost 8 in 10 Democratic-leaning adults (78 percent) say this question is a “serious issue” compared with under 1 in 7 (13 percent) of Republican leaners. That’s a 65-point gap.

Not everything about Mueller’s investigation produces a close split in public opinion, and several core aspects drew clear majority support in an April Post-ABC national poll. Nearly 7 in 10 supported Mueller investigating possible collusion between the Trump campaign and Russian government (69 percent).

That big partisan divide along with evidence that Republicans have grown more united in their criticism of the Mueller investigation after months of criticism by Trump and his allies suggest they have largely unified fellow partisans in skepticism toward the probe. The divisions are the scaffolding for a deeply partisan reception to whatever the investigation finds.

The Washington Post-Schar School poll was conducted June 27 to July 2 among a sample of 1,473 adults interviewed through the AmeriSpeak Panel, the probability-based panel of NORC at the University of Chicago. The survey oversampled adults in competitive congressional districts totaling 865 interviews. The error margin for overall results is plus or minus five percentage points and 4.5 points in battlegrounds.

Scott Clement contributed to this report.