Welcome to our Election Update for the Senate for Wednesday, Oct. 17.

If you’ve been following our Senate model — you should, everyone’s doing it — you may have noticed a trend of late: Democrats’ chances of winning the chamber have been diminishing. (We’ve written about it a bit.) Races in red states such as Texas and Tennessee seem to be pulling away from Democrats, perhaps in predictable partisan form. But Democrats have problems elsewhere too: The races in Arizona and Nevada — a couple of contests that were once supposed to be easier seat pickups for Democrats — have tightened. In both states, Democrats had been hoping that demographics and candidates well-suited to their political environments would help edge up the party’s Senate margins. But things aren’t going entirely to plan.

First let’s look at Arizona. Democrat Rep. Kyrsten Sinema is running against Republican Rep. Martha McSally to fill the seat of retiring Republican Sen. Jeff Flake. The classic version of FiveThirtyEight’s model currently gives Sinema a 3 in 5 chance of winning the seat, so the Democrat is still favored. But polls from September routinely showed her up on McSally by between 2 and 7 percentage points, while October polls have shown her lead dwindling. A new Siena College/New York Times survey conducted on Monday and Tuesday showed McSally leading by 3.8 points, while Democratic pollster Change Research showed the race essentially tied in a poll done Oct. 9-10.

Sinema is a moderate Democrat whose moderateness is in some ways well suited to the race — Arizona voted for President Trump, and Flake made his decision not to run in part because he felt his vocal opposition to the president left him with no viable electoral path forward in the state. McSally is an establishment Republican who didn’t endorse Trump in 2016 but who’s made efforts to laud the president throughout her campaign. With two moderate women running for Senate in a state that swung Republican in the last election, the race has fascinating fundamentals for a campaign of the Trump era.

Some of what might be behind the polls tightening on Sinema are ghosts of her political past. She was a vocal protester of the Iraq and Afghanistan wars, and she used to be a member of the Green Party. A CNN report last week rehashed some comments that she made on a radio show in 2003: During a back-and-forth with the libertarian host about hypotheticals, she said she wouldn’t mind if he decided to join the Taliban. In a debate this week, McSally called the comments “treason.”

Whatever the cause, Sinema’s chances (60 percent, to be precise) have edged down by a few percentage points compared with mid-September, when we launched our Senate forecast.

Then there’s Nevada, where Democratic Rep. Jacky Rosen is running against Republican Sen. Dean Heller. The FiveThirtyEight classic model currently gives Heller a 3 in 5 chance of keeping his seat. But the Republican senator was initially seen to be vulnerable because Hillary Clinton won the state in 2016 and because two weeks before that election, Heller declared that he was, “100 percent against Clinton, 99 percent against Trump.” The comment made Heller the target of Trump’s ire for a time — the president memorably sat next to the senator during a press conference and threatened his seat over a vote on health care. But the president and Heller have since made amends, and Trump has stumped for the senator this fall, with another rally planned in the state before Nov. 6.

Rosen, meanwhile, is a relative unknown — she was only just elected to the House in 2016. Polls from the spring, summer and early fall showed her consistently leading the race, but October polls have been purely bad news for Rosen, showing Heller in the lead. Trump’s attentions, it would appear, are still proving persuasive in a purple state. Heller has gone from a modest underdog — he had a 41 percent chance of winning when our Senate forecast launched — to a modest favorite (he’s at 59 percent now).

One other Senate race we’re still keeping our eye on is North Dakota. The state hasn’t been polled since a Sept. 29-Oct. 2 Fox News survey, which showed Democratic Sen. Heidi Heitkamp down by 12 points to her Republican challenger, Kevin Cramer. That’s the worst poll we’ve seen for Heitkamp. And new polls might well spell bad news for her too — she’s had a rough couple of weeks. On Oct. 6, she voted against the confirmation of Brett Kavanaugh — a risky political move given that Trump won her state handily. Then just this week, Heitkamp was forced to apologize for an ad about sexual assault run by her campaign that included the names of women who didn’t authorize their use or said they weren’t abuse survivors.

Check out all the polls we’ve been collecting ahead of the 2018 midterms, as well as our House and Senate forecasts.