The “Impeach Donald Trump Now” campaign, which called for the president’s impeachment for “serious violations” and “unprecedented level of corruption” since he took the office, has gained over half a million signatures.

The campaign, an independent public interest initiative, is led by nonpartisan and nonprofit Free Speech for People and RootsAction.

“As of January 20, 2017, President Trump’s refusal to divest from his business interests has placed him in direct violation of the U.S. Constitution’s Foreign Emoluments Clause and on a collision course with the Constitution’s Domestic Emoluments Clause and with the federal STOCK Act [Stop Trading on Congressional Knowledge Act of 2012],” according to the campaign’s website.

The campaign claims that Trump’s business holdings in the U.S. and across the world show conflicts of interest, which violate the American constitution. Ron Fein, legal director of Free Speech For People, on Tuesday alleged that Trump is making profit at public expense.

“On inauguration day, we issued the call for Congress to investigate whether President Trump should be impeached for violating the Constitution by holding onto his business interests,” Fein said in a statement. “Donald Trump is profiting from the presidency at public expense, and people are suffering as a result.”

On Jan. 23, Citizens for Responsibility and Ethics in Washington, a liberal legal watchdog, sued Trump accusing him of breaching the country’s constitution by letting his businesses receive payments from foreign governments. The lawsuit alleged the 70-year-old president accepted cash and favors from foreign governments through guests and events conducted at his hotels, leases in his buildings and real estate deals across the world.

The American constitution allows a president to be impeached for “treason, bribery, or other high crimes and misdemeanors.” However, the process is complex. It begins in the House of Representatives. Following this, it goes to the Senate where a vote is conducted to remove the president from office. While the House needs a majority vote, the Senate requires a two-thirds vote to impeach the president. If Trump is ousted from office through this process, Vice President Mike Pence will take his place.

There has never been an impeachment by both the House and Senate. However, some presidents have come close to getting impeached. Richard Nixon stepped down from his office, paving way for Gerald Ford to take office in 1974. And in 1998, Bill Clinton was impeached by the House but not the Senate.