President Donald Trump once again targeted House Democrats who are conducting a formal impeachment inquiry over the Ukraine whistleblower scandal in tweets accusing them of treason on Sunday, calling for the impeachment of House Speaker Nancy Pelosi and Intelligence Committee Chairman Adam Schiff.

"This makes Nervous Nancy every bit as guilty as Liddle’ Adam Schiff for High Crimes and Misdemeanors, and even Treason. I guess that means that they, along with all of those that evilly “Colluded” with them, must all be immediately Impeached!" Trump said.

Pelosi and Schiff are spearheading the Trump impeachment investigation over concerns that he improperly pressured Ukraine's president to investigate political rival Joe Biden. Trump also has suggested that Sen. Mitt Romney, R-Utah, face impeachment.

But can a member of Congress be impeached? According to law, no, but there are other steps that can be taken.

More:Donald Trump now accuses Nancy Pelosi of 'treason' (and wants to impeach her)

What the law says about impeachment

Impeachment is laid out by the Constitution as the first step in the process to remove an official from office. A president, vice president, or "civil officer" can be impeached.

The House of Representatives holds the authority to vote on and submit articles of impeachment to the Senate, which then has the power to preside over an impeachment trial and ultimately vote on removal from office.

Impeachment is a step that can be taken against an official appointed by the president, including members of the president's Cabinet and judges.

More:How does a Supreme Court justice get impeached?

House Democrats are leading the impeachment probe into the president to uncover more details about his dealings with Ukraine, and whether he withheld financial aid to the country in exchange for an investigation into Biden and his son, Hunter Biden.

Trump has made the allegation without evidence that the former vice president pressed Ukraine to fire a top prosecutor who was looking into corruption in a company where his son once sat on the board.

Explainer:What really happened when Biden forced out Ukraine's top prosecutor

Can impeachment be used against members of Congress?

Technically, members of Congress do not go through impeachment proceedings. But they can be expelled from office and disciplined in other ways.

According to a report by the Congressional Research Service, expulsion is a more simplified process than impeachment.

"A removal through an impeachment requires the action of both houses of Congress – impeachment in the House and trial and conviction in the Senate; while an expulsion is accomplished merely by the House or Senate acting alone concerning one of its own Members, and without the constitutional requirement of trial and conviction," the report states.

House and Senate members can expel members with a two-thirds vote, as is laid out in Article I of the Constitution.

A congressional recall, on the other hand, is not provided for by the Constitution. This consists of constituents voting to place an early limit on the term of their representative through a special election. Some states do provide for recalls for some other state officials, but Supreme Court decisions have opposed the right of states to impose a recall on members of Congress, according to the CRS report.

Another way to discipline a members of Congress, but that would not result in removal from office, is a censure, which is a formal reprimand of a member that requires a simple majority vote.

A House Republican has also introduced a motion to censure Schiff over his exaggerated reenactment of a phone call between Trump and the Ukraine president.

So, Trump's call for the impeachment of his critics won't pan out for legal reasons, and it would also be difficult to expel them from office.

Has the removal of a member of Congress ever been tried?

There is only one documented case of an attempted impeachment of a member of Congress, in 1797. Though the House of Representatives voted to impeach Sen. William Blount, the Senate rejected the move for reasons including that he was not considered a "civil officer" based on his role in Congress and that he had already been expelled, which made impeachment a moot point.

As for expulsion, 15 senators and five members of the House have met that fate in the history of the United States. Most were related to Civil War infractions. The United States hasn't seen an expulsion since 2002, when Ohio Rep. James Traficant was convicted on bribery, racketeering and tax evasion charges.