Thanks to big shifts in several key races, Democrats now have a 73 percent chance of winning the Senate, according to the FiveThirtyEight polls-plus forecast, and a 72 percent chance according to polls-only. Both those numbers are up by more than 15 percentage points from last week, when the polls-plus model gave them a 56 percent chance and the polls-only model 54 percent.

In our last Senate update, I noted a growing divergence between our Senate and presidential forecasts. The presidential race had moved quickly toward Democrat Hillary Clinton; the battle for control of the Senate had ticked a bit toward the GOP. But the forecasts have snapped back into closer alignment as Democratic Senate chances have improved.

What’s so interesting about the upswing in Democrats’ fortunes is that many Senate races haven’t shifted much. In the average Senate race, the margin separating the two major-party candidates has shifted toward the Democrat by just over 1 percentage point over the last week, according to our polls-only model. But that includes noncompetitive races. Control of the Senate is coming down to six key states, with Democrats needing to gain four seats to win a majority if Clinton wins the White House. And in the crucial contests, there has been more movement.

Democratic wins in Illinois and Wisconsin look likely, so they need to win three of these six races: Indiana, Missouri, Nevada, New Hampshire, North Carolina and Pennsylvania. These six states top our tipping-point rankings, which measure the chances of each race deciding control of the Senate.

Now look at how the Democrat’s chances and projected margin of victory has changed over the last week in each of these six states.

DEMOCRATIC MARGIN CHANCE OF WINNING RACE OCT. 12 OCT. 20 CHANGE OCT. 12 OCT. 20 CHANGE Missouri -3.0 +1.0 +4.0 33.4% 56.6% +23.2 New Hampshire -1.6 +1.3 +2.9 39.6 59.1 +19.5 Nevada -3.2 0.0 +3.2 31.6 50.7 +19.1 North Carolina -1.6 -0.7 +0.9 39.0 44.3 +5.3 Pennsylvania +0.5 +0.9 +0.4 52.8 56.3 +3.5 Indiana +7.5 +6.4 -1.1 82.3 82.5 +0.2 How key Senate races have shifted over the last week Based on FiveThirtyEight’s polls-only forecast, as of 7 p.m. on Oct. 20

The news is very good for Democrats. The Democrat has a better chance than they did a week ago in all six states, although just barely in Indiana. Indiana, though, isn’t the big story. In Missouri, New Hampshire and Nevada, Democratic chances of winning are up dramatically.

For the first time all year, Democrat Jason Kander is slightly favored to defeat Republican Sen. Roy Blunt in Missouri. The model doesn’t have a ton of data to work with in Missouri. We’ve only had three polls conducted over the last month (and only one is a live interview survey), but the average of the three gives Kander a small advantage. Before mid-September, Kander had trailed in every poll. While Missouri is a red state on the presidential level, it has a history of supporting Democrats for statewide offices. The current governor is a Democrat and is favored to be replaced by a Democrat. Missouri’s other U.S. senator, Claire McCaskill, is a Democrat and was re-elected in 2012, even as Mitt Romney was easily winning the state. Given this history and Kander’s strong campaign, there’s no particular reason to doubt that Kander has a small lead.

Democrat Maggie Hassan’s chances are also up in New Hampshire. While the race in New Hampshire has been tight all year, Hassan received one of her best polls of the campaign this week. A University of New Hampshire poll released Thursday evening showed her leading Republican Sen. Kelly Ayotte by 9 percentage points. While it’s too early to know if this poll is an outlier or the start of a new trend, Ayotte has had a difficult time figuring out how to deal with Trump. She called him a role model during a debate but retracted that afterward. Now, she won’t even vote for him. Trump, of course, remains a heavy underdog in the Granite State.

The other big mover is Nevada. Sen. Harry Reid’s hand-picked successor, Catherine Cortez Masto, has been trailing Republican Joe Heck most of the year. She had led in only three polls out of 21 released before Oct. 10. Since that time, though, she’s posted three leads and has led in the average poll by a little over 1 percentage point. Cortez Masto may be benefiting from Clinton’s growing advantage in the state. Heck, meanwhile, has struggled in appeasing Trump voters while distancing himself from the Republican nominee for president. Although the race remains tight, remember that four years ago in Nevada, Republican Dean Heller won his race by just 1 point, despite facing off against a scandal-ridden Democrat, Shelley Berkley. Cortez Masto doesn’t have the same baggage Berkley did.

All three of these races remain close, as do the other two big contests: North Carolina and Pennsylvania. Democrat Katie McGinty and Republican Pat Toomey in Pennsylvania continue to trade polling leads, while Republican Richard Burr’s advantage over Democrat Deborah Ross in North Carolina has been slowly shrinking. Those races look likely to go down to the wire.

The thing to remember, though, is that if Illinois and Wisconsin stay safely blue and Evan Bayh continues to maintain an edge in Indiana, Democrats only need to win two of the other five close races. And they’re now favored (if only slightly in most) in more of those races than Republicans are. You can see why Democrats now have about the best shot of winning back the Senate that they have had all year.

CORRECTION (Oct. 24, 10:33 a.m.): A previous version of this post incorrectly described the status of the race for the U.S. Senate seat in Indiana. The margin of the Democratic candidate, Evan Bayh, over his Republican opponent was slightly down over the previous week, while his chances of winning increased; Bayh’s polling number was not higher than it was last week.