The US presidential election will be held on November 5. In analyst Nate Silver’s aggregate of national polls, Democrat Kamala Harris leads Republican Donald Trump by 48.8–45.0. Last week, Harris led Trump by 48.8–44.8.

Joe Biden’s final position before his withdrawal as Democratic candidate on July 21 was a national poll deficit against Trump of 45.2–41.2. By the election, Biden will be almost 82, Trump is now 78 and Harris will be 60.

There were two developments last week that potentially changed the polls. First, the Democratic National Convention was held from August 19–22. Second, Robert F Kennedy Jr withdrew from the presidential contest and endorsed Trump on August 23. When he withdrew, Kennedy had only about 4 per cent support in national polls.

The next event that could potentially change the race is the September 10 debate between Harris and Trump. The June 27 debate between Biden and Trump eventually led to Biden’s withdrawal.

Pennsylvania a problem for Harris

The US president isn’t elected by the national popular vote, but by the Electoral College, in which each state receives electoral votes equal to its federal House seats (population based) and senators (always two). Almost all states award their electoral votes as winner takes all, and it takes 270 electoral votes to win (out of 538 total).

In Silver’s model, Harris’ chance of winning the Electoral College has fallen from 53.2 per cent last Sunday to 47.3 per cent, with Trump now the favourite to win at 52.4 per cent probability. Harris’ win probability peaked on August 14 at 57 per cent. This is the first time Trump has been favoured to win since August 3.

The Electoral College is biased towards Trump relative to the national popular vote. In Silver’s model, Harris needs to win the popular vote by at least two points to be the Electoral College favourite.

A key reason for Harris’ drop is Pennsylvania, which has 19 electoral votes, the most of any of the swing states. If either Harris or Trump wins Pennsylvania, that candidate has about a 92 per cent chance to win the Electoral College.

Silver wrote that recent polls in Pennsylvania have been weak for Harris, although his model does not yet include a Morning Consult poll that gave Harris a four-point lead in Pennsylvania.

I previously wrote that, from the point of view of maximising her chances to win the Electoral College, Harris should have selected Pennsylvania Governor Josh Shapiro as her running mate instead of Minnesota Governor Tim Walz. Democrats could regret this choice.

Another factor that is hurting Harris is the model’s convention bounce adjustment. Convention bounces usually peak a week after the convention, then fade in the next few weeks. I previously wrote that Harris needed to be leading by four to five points nationally in polls conducted after the Democratic convention to maintain her win probability.

Harris’ national lead of 3.8 points is just below the lower bound where she would maintain her win probability. If her national lead is unchanged in the next two weeks, her win probability will increase again. But the model expects Harris’ lead to drop in the next two weeks.

Further analysis

It’s possible that Kennedy’s withdrawal and endorsement of Trump has changed some of his voters into Trump voters. In most polls before Kennedy’s withdrawal, Harris benefited from his inclusion compared with a Harris vs Trump head to head version.

The initial surge for Harris was probably driven by relief at Biden’s withdrawal. Biden now has a net approval of -12.9 in the FiveThirtyEight aggregate, an improvement from -17.1 when he withdrew on July 21. But his unpopularity is still likely to hurt Harris, who is campaigning on a continuation of Biden’s policies.

After large gains in Harris’ net favourability in the first two weeks of her candidacy, her net favourability gains have stalled. Harris’ net favourability in the FiveThirtyEight tracker is -1.4, up from -16.0 on July 20, but a much smaller improvement from -3.9 on August 14.

I believe Harris’ honeymoon from replacing Biden is over, and it will be harder for her to make gains in polls now.

Trump’s net favourability has dropped back to -9.2 after gaining some ground following the mid-July Republican convention. Trump’s running mate JD Vance is at -9.8 net favourable, while Walz is at +4.8.

Adrian Beaumont, election analyst (psephologist) at The Conversation; and Honorary Associate, School of Mathematics and Statistics, The University of Melbourne.  Republished from The Conversation.