WASHINGTON — Leading Democratic senators are expected to introduce a constitutional amendment Tuesday to abolish the Electoral College, adding momentum to a long-shot idea that has been gaining steam among 2020 presidential candidates.

Sen. Brian Schatz of Hawaii plans to introduce the measure along with Sen. Dick Durbin of Illinois, the No. 2-ranking Democrat in the Senate, and Sen. Dianne Feinstein of California, the top Democrat on the Senate Judiciary Committee, according to Schatz's spokesperson.

Also signed on to the legislation is Sen. Kirsten Gillibrand of New York, one of a growing number of presidential candidates who have called for electing presidents by popular vote, even though changing the Constitution is seen as virtually impossible today.

A constitutional amendment may be proposed by a two-thirds supermajority in both the House (about 290 votes) and Senate (67 votes) and requires ratification by 38 states.

Sens. Elizabeth Warren of Massachusetts, Kamala Harris of California, Bernie Sanders, an independent from Vermont, former Rep. Beto O’Rourke of Texas, Pete Buttigieg, the Democratic mayor of South Bend, Indiana, and former Housing and Urban Development Secretary Julián Castro are among the presidential candidates who have expressed openness to abolishing the Electoral College.

“We should abolish the Electoral College,” Castro said at a 2020 Democratic candidate forum in Washington on Monday. “It doesn't reflect the will of the people of the country.”

Two of the last three presidents have lost the popular vote but won the Electoral College, leading critics to charge that the system is undemocratic and threatens the legitimacy of the American political system.

Those calls have been led primarily by Democrats, since they lost out in both 2000, when George W. Bush beat popular vote winner Al Gore, and 2016, when Donald Trump beat Hillary Clinton even though she had almost 3 million more votes.

The current system assigns electoral votes on a state-by-state basis according to the size of their congressional delegations, which gives an advantage to smaller states and a disadvantage to bigger states since all states have the same number of senators.

Defenders of the Electoral College say it protects the interests of smaller and more rural states, and warn that presidential candidates would only campaign in major population centers such as New York City and Los Angeles if the popular vote alone mattered.