One of the Democratic party's biggest donors, billionaire activist Tom Steyer, is calling on Democrats in Congress to fight for the impeachment of President Trump and to declare their position on impeachment.

In a letter obtained by CBS News, arguing that Mr. Trump should be impeached because he has "violated the public trust" and has "clearly already" exceeded the standards historically used by Congress for impeachment, Steyer argues that the views taken by Democrats on the issue should be disclosed to voters before the 2018 midterm elections.

"Given that it is clear for all to see that Trump is not fit for office, the question of impeachment becomes a very real issue when Democrats take back the House and Senate, which makes it imperative that the public deserves to know where every Democrat stands on the issue of the highest import to the lives of every single American now, before those elections happen," Steyer wrote to Democrats in the House and Senate.

Get Breaking News Delivered to Your Inbox

"This is not," Steyer said, "a time for 'patience' -- Donald Trump is not fit for office. It is clear for all to see that there is zero reason to believe "he can be a good president." This was a slap at California Sen. Dianne Feinstein, who in August called for "patience" with Mr. Trump's presidency, rather than impeachment, when she was asked about it at an event in San Francisco. At the time, she said that he was "going to be president most likely for the rest of this term," and added, "I just hope he has the ability to learn and change. If he does, he can be a good president."

There is little chance of removing the president from office by impeachment, given that Republicans control Congress, and ultimately, two-thirds of the Senate would have to vote for Mr. Trump's removal.

The New York Times first reported the letter. Read it here:

Steyer is thought to be considering whether to challenge Feinstein in 2018 or run for California governor when Jerry Brown's term is up, also in 2018. He considered running for Barbara Boxer's seat in 2016 but ultimately decided against it.

CBS News' Steve Chaggaris contributed to this report.