WASHINGTON — The last time a president-elect lost the national popular vote was in 2000, when George W. Bush narrowly beat Al Gore in the Electoral College vote. The three times before that were more than 100 years ago. Now, President-elect Donald Trump’s 306-232 electoral-vote victory over Hillary Clinton can be added to the list, which has prompted increased efforts to base presidential victories on the popular vote.

The nation’s quadrennial interest in moving toward a national popular vote is gaining traction this time, fueled by the fact that Clinton is likely to have a more than 2 million-vote lead when all voters’ ballots are tallied.

National Popular Vote, an organization founded in 2005 to push state legislatures to order their electoral votes to go to the popular vote winner, has seen a spike in hits to its website – now at 250,000  a day –  and more than 100,000 people have used a feature to write to their lawmakers. Dozens of petitions to end the current electoral-vote system have sprouted on Change.org.

In a symbolic move, Sen. Barbara Boxer, a California Democrat who is retiring this year, introduced a bill a week after the election to abolish the Electoral College – a bill that a Republican-controlled lame-duck congressional session, is very unlikely to consider.

“In my lifetime, I have seen two elections where the winner of the general election did not win the popular vote,” Boxer said in a statement. “The Electoral College is an outdated, undemocratic system that does not reflect our modern society, and it needs to change immediately. Every American should be guaranteed that their vote counts.”

Going back as far as 1944, public polling has indicated a majority of Americans — at levels usually greater than 60 percent — support changing the presidential election system to a national popular vote.

But efforts to change the way presidential winners are determined face considerable obstacles. For those who want to abolish the Electoral College, like Boxer, it would take a constitutional amendment. That would require three-quarters of states to ratify the change.

Another way to change the current presidential election system is through an interstate compact. National Popular Vote maintains this is the constitutionally appropriate way, while opponents criticize the movement for finding an “end-run to the constitutional amendment process.”

Instead of pushing for a constitutional amendment, National Popular Vote calls on state legislatures to require the state’s electors to cast their votes based on the winner of the national popular vote. The compact would not take effect until states representing at least 270 electoral votes have signed it.

But critics say that there are constitutional hurdles to overcome before a national popular vote system could see the light of day, even if enough states signed on. And that’s another obstacle — whether states from both sides of the political spectrum will sign on, especially red states.

“The Republicans have lost the popular vote for president every year but one since 1988, but they’ve elected a couple of presidents,” said Alex Keyssar, a professor of history and social policy at Harvard University. “I think that’s the biggest obstacle right now.”

The winner-takes-all system for allocating electoral votes that 48 states use is not required by the Constitution. National Popular Vote supporters — as well as Maine and Nebraska, which award electors by congressional districts — rely on Article II of the document, which gives state legislatures the authority to determine how electors are allocated.

“There’s this false narrative out there that somehow, some way the system we have in place right now is the Electoral College system,” said Pat Rosenstiel, senior consultant to National Popular Vote. “No, it’s how the states have chosen to award electors.”

Legislation to join the compact has been adopted by 10 states and the District of Columbia, totaling 165 electoral votes. The bill has passed in one legislative chamber in other states that represent 96 electoral votes. If those are all successful, that would put the number of electoral votes at 261, just nine shy of the magic number of 270.

Rosenstiel recognized those that have signed on — California, New York, Illinois, to name a few — are traditionally blue, but there is evidence of bipartisan support. The most recent legislative chambers to pass the bill were the Arizona House of Representatives and the Oklahoma Senate, both Republican-controlled bodies in red states. The National Popular Vote bill has also been passed in the New York legislature. Although New York is a blue state, the the Republican-controlled Senate passed the bill, which then also was approved by the Democrat-controlled House.

Jack Rakove, a professor of political science and history at Stanford University, said Republicans should remember that had 60,000 votes in Ohio swung to Bush’s Democratic opponent in 2004, John Kerry would have been the president-elect, and Republicans would have found themselves in the situation Democrats currently find themselves in. Bush won re-election that year with a nearly 3-million vote margin.

“Perhaps that’s too bad,” Rakove said, “because had both parties been victimized by this, they might have said, ‘Well, the fair way to do it is to just have a national vote.’”

Keyssar, the Harvard professor, said a bill to ask states to ratify a vote for a constitutional amendment enacting a national popular vote got close to congressional approval in 1969. With overwhelming support in the House, it went to the Senate, where a majority of senators supported it. But there weren’t 60 votes to break a filibuster that killed the attempt.

Tara Ross, a retired lawyer and author of “Enlightened Democracy: The Case for the Electoral College,” sees the compact as a way to circumvent  the constitutional amendment process, which she believes would fail. . National Popular Vote proponents argue that a constitutional amendment would strip states of their constitutional authority to award electors. Even if the compact received enough support, there is a question of whether congressional approval is required before it could take effect because the Constitution says interstate compacts require congressional consent. Courts have ruled, however, that congressional approval is only necessary for compacts that would threaten federal supremacy.

Even Rakove, who supports abolishing the Electoral College, has doubts about the constitutionality of an interstate compact “to fundamentally alter the nature of our political system.” He also said the compact idea would create political instability because a future legislature could rescind support for the compact given by an earlier one.

Guy-Uriel Charles, a Duke Law School professor who specializes in election and constitutional law, said there are no constitutional problems with the National Popular Vote proposal. The compact does not abolish the Electoral College, but “is using constitutional mechanisms that authorizes the states to either say how Electoral College votes ought to be allocated or constitutionally permitting the states to bind Electoral College electors,” he said.

Charles also questions whether the agreement among the states is truly a “compact,” as defined by the Constitution. Technically, states are not agreeing to do anything; rather, they are simply saying they will participate if others do.

Rosenstiel is betting that enough legislatures will join the compact to change the way electoral votes are allocated in 2020.

“There would be no concentration of resources, attention and power in the hands of battleground state voters. Every voter in every state would be a battleground voter, because whoever wins the most popular votes in all 50 states would win,” he said. “You end up with a single-member district: the United States of America.”