Nevada's Democratic governor on Thursday vetoed a measure that would have automatically pledged the state's six Electoral College votes to the presidential candidate who wins the national popular vote, in a major stumble for Democrats' nationwide effort to enhance their chances ahead of the 2020 election.

Gov. Steve Sisolak - who earlier this year was sworn in as the first Democratic governor in Nevada in more than two decades -- warned that the proposal "could diminish the role of smaller states like Nevada in national electoral contests."

“I recognize that many of my fellow Nevadans may disagree on this point and I appreciate the legislature’s thoughtful consideration of this important issue," Sisolak said in a statement.

HOWARD KURTZ: WHY DEMS WANT TO ABOLISH THE ELECTORAL COLLEGE

Nevada went for Hillary Clinton over Donald Trump by fewer than three percentage points in 2016.

Democrats, who currently have a stranglehold on power in population-dense states like California and New York, have long protested the Electoral College. States receive electoral votes equivalent to their number of congressional districts plus senators, which allows less populous states to have more impact than they would under a popular vote system. The upcoming 2020 census is expected to result in some shifts in Electoral College numbers by 2024, including an increase in electoral votes for traditional GOP strongholds like Texas.

The so-called National Popular Vote interstate compact, which would commit states' electors to the winner of the national vote, aims to neutralize the Electoral College before then. The compact has been adopted by fifteen jurisdictions accounting for 189 electoral votes, including fourteen states and the District of Columbia.

The states include Rhode Island, Vermont, Hawaii, Delaware, Maryland, Massachusetts, New Mexico, Washington, Connecticut, Colorado, New Jersey, Illinois, California, and New York.

However, the compact, by its terms, will only take effect if jurisdictions accounting for at least 270 of the 538 total votes available in the Electoral College sign on. Currently, that would require states with 81 more electoral votes to agree to the compact.

Parallel congressional efforts have been unsuccessful. In January, Rep. Steve Cohen, D-Tenn., introduced a pair of constitutional amendments to eliminate the Electoral College, saying it was "outdated."

ANALYSIS: IN DEFENSE OF THE ELECTORAL COLLEGE

“Americans expect and deserve the winner of the popular vote to win office,” Cohen said at the time. “More than a century ago, we amended our Constitution to provide for the direct election of U.S. Senators. It is past time to directly elect our President and Vice President.”

However, a constitutional amendment eliminating the Electoral College would require two-thirds of both the House and Senate to approve the measure, along with three-fourths of state legislatures. Alternatively, Congress could hold a national convention and states could host ratifying conventions, but a two-thirds majority would still be necessary.

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Trump secured victory in the 2016 election by winning the Electoral College with 304 votes to Democrat Hillary Clinton’s 232 despite Clinton winning nearly three million more votes than Trump.

John Quincy Adams, Rutherford B. Hayes, Benjamin Harrison, and George W. Bush also won the White House without winning the popular vote. Of those presidents, only Bush was re-elected to a second term.