President Donald Trump’s approval rating has dropped to its lowest point since the inauguration, and support among his Republican base appears to be slipping. In April, 43 percent of Americans polled approved of the president’s job performance. Now that number has dropped to 36 percent, according to the latest CBS News poll.

Disenchanted Republicans are partially responsible for that dip — a major sign that support from Republican voters is starting to erode. During his first months in office, Trump appeared to do no wrong in the eyes of his core supporters, who have consistently given him high ratings despite endless Russia-related scandals and lack of progress on his legislative agenda. After his first 100 days in office, 83 percent of Republicans polled still approved of Trump’s job performance. That number has now dropped to 72 percent.

The president’s handling of the Russia investigation seems to be most corrosive to his approval rating, based on questions about how Trump is handling the economy, terrorism, and the Russia probe. About 63 percent of Americans polled disapprove of how the president has handled the Justice Department’s investigation into potential ties between his campaign and the Russian government — more so than almost anything else he has done as president. And news that Trump is now under investigation for possible obstruction of justice related to his firing of former FBI-Director James Comey probably didn’t help either.

But while Republicans are more critical now of the president’s actions related to the investigation, they don’t seem to think the investigation itself is such a big deal. More than half of Republicans think the Russia investigation is a political distraction, and only one in five consider it a critical security issue. Even among Republicans who think it’s possible that the president’s campaign had improper contact with the Russian government, only a third view it as a serious matter of national security.

Despite differing views on the gravity of the investigation, more than half of Americans trust that the special counsel’s probe will be impartial, despite assertions by Trump’s allies that the head of that team, former FBI Director Robert Mueller, will be biased toward Comey. Recent news reports, quoting White House officials, have said that Trump was thinking about firing Mueller.

Based on the polls, that would almost definitely be a bad move. An overwhelming number of Republicans, Democrats, and independents all seem to agree on one thing: Trump should not try to stop the investigation.