President Trump's call for his former rival Hillary Clinton to be prosecuted is an "impeachable offense," former President George W. Bush's chief ethics lawyer said Tuesday.

"Pressuring AG to prosecute the person who lost the election is an impeachable offense if we value free elections," Richard Painter tweeted.

Pressuring AG to prosecute the person who lost the election is an impeachable offense if we value free elections. https://t.co/3J78u9ucZb — Richard W. Painter (@RWPUSA) July 25, 2017

"Congress must act now," he added in a subsequent tweet.

A President who can get his AG to prosecute a person who lost the election will put an end to free elections. Congress must act now. https://t.co/czpejQTrx5 — Richard W. Painter (@RWPUSA) July 25, 2017

Trump lashed out at his attorney general for a second day in a row in an early morning tweetstorm. Trump said Sessions, who was the first senator to announce his support for Trump, has taken "a VERY weak position on Hillary Clinton crimes."

Trump told the New York Times in November that he didn't feel "very strongly" about following through on his campaign promise to investigate Clinton. But Trump appears to have changed his mind as the investigation into Russia's meddling in the 2016 election heats up.

On Saturday, Trump said "so many people" were asking why Sessions or the special counsel investigating Russia's interference weren't probing Clinton or former FBI Director James Comey's "crimes."

In a tweet Monday morning, Trump asked why the "beleaguered" attorney general wasn't investigating "Crooked Hillarys crimes & Russia relations."

Sessions recused himself from the investigation after it was revealed that he met with Russia's ambassador to the U.S. during the 2016 presidential campaign.

At his confirmation hearing, Sessions said he would recuse himself from any matters dealing with the investigation into Clinton's use of a private email server while she was secretary of state because he had made several comments on the matter while campaigning for Trump.