Elizabeth Warren wrote a post on Facebook backing Betterment's CEO. Facebook screenshot. Democratic Sen. Elizabeth Warren is backing the CEO of a hot new finance startup.

The Massachusetts senator wrote on her Facebook page on Tuesday that she agrees with Betterment CEO Jon Stein, who wrote an open letter to President-elect Donald Trump about keeping the new fiduciary rule.

Her post linked to Business Insider's Monday story about Betterment's letter to Trump.

"I agree with CEO Stein," Warren wrote. "Undercutting the rule would be bad for working families — and bad for the many businesses who have already complied with the new rule."

Warren has long backed the fiduciary standard, which the Department of Labor passed earlier this year and which makes it much harder for conflicted financial advisers to work in their own interests over their clients' when managing retirement money.

Betterment, which provides index funds, bought full-page ads in the print editions of The Wall Street Journal and The New York Times earlier this week urging Trump to uphold the rule.

The rule is expected to financially benefit firms like Betterment since advisers would be more likely to put client assets in passive funds, which are cheaper than actively managed funds.

While Trump hasn't indicated his view, one of his advisers, hedge funder Anthony Scaramucci, has said Trump would repeal it.

Some Wall Street firms, such as Morgan Stanley, are preparing for the rule to be delayed in its start date, which is planned for April.

In a research note on Monday, UBS wrote that it is "more confident" in its expectation of a delay.

The note also said that Morgan Stanley has "developed alternate plans" in case it doesn't have to comply with the new rule.