FILE - In this Oct. 14, 2016 file photo, Democratic presidential candidate Hillary Clinton speaks to volunteers at a campaign office in Seattle. Hillary Clinton has a tight grip on the Electoral College majority need to be elected president of the U.S., and may very well be on her way to a big victory, and that's how some Republicans see it. (AP Photo/Andrew Harnik, File)
FILE - In this Oct. 14, 2016 file photo, Democratic presidential candidate Hillary Clinton speaks to volunteers at a campaign office in Seattle. Hillary Clinton has a tight grip on the Electoral College majority need to be elected president of the U.S., and may very well be on her way to a big victory, and that's how some Republicans see it. (AP Photo/Andrew Harnik, File)

Washington — Hillary Clinton holds a decisive advantage over Donald Trump in the competition for votes in the electoral college, leading in enough states to put her comfortably over the 270 majority needed to win the presidential election in November, according to a new SurveyMonkey poll of 15 battleground states conducted with The Washington Post.

Based on the results from the 15 state surveys, along with assumptions of the likely outcomes in other states that have consistently voted for one party or the other, Clinton, the Democratic nominee, holds leads of four percentage points or more among likely voters in states that add up to 304 electoral votes.

Trump, the GOP nominee, has the advantage in states with an estimated electoral vote total of 138. Arizona, Florida, Ohio and Texas, which account for 96 electoral votes, remain as toss-ups. All results in the 15 state surveys are based on ballot tests that include Libertarian Party nominee Gary Johnson and Green Party nominee Jill Stein.

The results underscore the importance for Trump in tonight’s final presidential debate in Las Vegas. National polls have moved in Clinton’s direction since the exchanges began in late September. Her current average margin is seven points in polling averages from the Huffington Post Pollster and RealClearPolitics. The most recent Washington Post-ABC News poll put her national lead over Trump at four points.

The effect of the shift toward Clinton in national polls is evident in the new 15-state study. In late August, The Post, using SurveyMonkey’s online methodology, conducted individual polls in all 50 states among registered voters. At that time, Clinton led in states that added up to 244 electoral votes, while Trump led in states accounting for 126. Toss-up states equaled 168 electoral votes.

The SurveyMonkey surveys also included polls of Senate and gubernatorial races, where applicable, in the same 15 states. The results show two things. First, many Republican Senate candidates are outperforming Trump in their states. Second, that still may not be enough to maintain the GOP’s majority in the chamber.

Democrats need a net gain of five seats to take outright control of the Senate, four to exercise control if Clinton becomes president. The results showed Democrats with leads of four or more points in three states with GOP incumbents: New Hampshire, North Carolina and Wisconsin.

Republican incumbents lead in Florida and Ohio, in the latter by a wide margin. Meanwhile, races in Nevada, for a seat currently held by Democrats, and Pennsylvania, for a seat now in Republican hands, continue to be close. Pennsylvania is emblematic of the problem for GOP Senate incumbents trying to survive in states where Trump is struggling. Clinton leads Trump by six points in Pennsylvania, but the Senate race is even at 47 percent apiece for Republican incumbent Patrick Toomey and Democrat Katie McGinty.

The 2016 campaign has produced apparent shifts in several states — battlegrounds and non-battlegrounds — that could foreshadow more permanent changes in future elections. Demographic trends have moved several swing states more strongly in the direction of the Democrats — Colorado and Virginia are prime examples.

States such as Arizona, Georgia and possibly Texas show predicted signs of shifting from traditional GOP strongholds to potential future battlegrounds. The demographic changes would put Republicans at an even greater disadvantage in the future if they cannot find a way to appeal to a changing electorate.

At the same time, the electoral map looks somewhat different from past campaigns, owing to the uniqueness of Trump’s candidacy — the coalition he has attracted and the voters who are repelled by his message and temperament. His relative strength in states with high percentages of non-college graduates and of white voters, such as Iowa and Ohio, is an example of the former. Utah, which has been scrambled this year by Trump’s candidacy, is an example of the latter.

Overall, the results of the 15-state survey underscore Clinton’s wider margin in the state-by-state competition over the final three weeks of the campaign, as well as the increasingly tortuous path for Trump to cobble together an electoral-college majority.

The SurveyMonkey results also put Clinton in a stronger position in New Hampshire and North Carolina than other public surveys. Public polls in North Carolina show her with a low single-digit advantage. In New Hampshire, public polls show her ahead but not by the 11-point margin in these findings.