(CNN) There is a truly remarkable number in the most recent CNN poll, conducted by SSRS and out this morning.

In it, 42% of Americans say President Donald Trump should be impeached and removed from office.

What makes it remarkable is that he's on par with President Richard Nixon, who 43% of Americans said should be impeached and removed from office in a March 1974 Harris poll. That was after the scale of Watergate came to light, but months before the House started to move against Nixon, who would go on to resign in August 1974 rather than be impeached.

Impeachment requires "treason, bribery or other high crimes and misdemeanors," according to the Constitution, which also lists it as the the only thing for which a President can't issue a pardon.

Trump has bragged that he certainly has the power to pardon himself but won't need to use it. Nixon got a pardon from Gerald Ford, the man to whom he gave the keys to the White House.

The 43% supporting Nixon's impeachment in that Harris poll, by the way, is much higher than the 29% who supported impeachment for President Bill Clinton in 1998. Or, for that matter, the similar number who wanted Presidents Barack Obama and George W. Bush impeached. As CNN's Grace Sparks writes, there's basically "a baseline of pro-impeachment sentiment for a modern president" and Trump far eclipses it.

So why aren't top Democrats clamoring to impeach Trump? To be sure, there are efforts on the outskirts of the party and in Congress, but they do not have the backing of party big wigs.

Most Democrats in Congress, for the record, have opposed efforts by Rep. Al Green, a Texas Democrat, to bring the issue to the floor of the House.

"I do not think that impeachment is a policy agenda," House Minority Leader Nancy Pelosi, a California Democrat, said at a CNN town hall in May.

There's also the fact that it can't feasibly happen right now. Trump is the head of the Republican Party, in name and in practice. The Republican majorities in the House and Senate, the same ones that have supported his policy initiatives, are going to laugh at an impeachment effort, which requires a supermajority.

Congress can't get a supermajority for a simple bill to stop taking children from undocumented parents at the border. A supermajority on impeaching Trump is not happening.

Pelosi's counterpart in the Senate, New York's Sen. Chuck Schumer, has likewise said he'll wait for a "thorough investigation" before entertaining any impeachment talk.

"We need to get all the facts and then we'll come to conclusions," he said.

"Impeachment is, to me, divisive," Pelosi added back in May, although she suggested her view might change depending on what special counsel Robert Mueller finds. "Again, if the facts are there, if the facts are there, then this would have to be bipartisan to go forward. But if it is viewed as partisan, it will divide the country, and I just don't think that's what we should do."

It would be divisive: 87% of Republicans and people who lean toward Republicans oppose the idea, while 71% of Democrats support it.

Pelosi can easily remember the impeachment effort against Clinton, which royally backfired on Republicans since he survived a trial in the Senate and maintained his approval rating, which was over 70% at the time of his impeachment.

Trump's approval rating is much lower -- 39% -- in that new CNN poll.

An impeachment effort would also play right into the "witch hunt" and "deep state" narratives Trump has been pushing.

Schumer and Pelosi might have to form an opinion sooner than they'd like: Trump's lawyer Rudy Giuliani has said Mueller should wrap up his investigation by September, which is just before the midterm elections in November, when Democrats would like to take control of the House and Senate.