Donald Trump’s post-conventions polling slump seems to be having an effect on the Republican Party’s U.S. Senate candidates. We thought this might happen: There’s been an increasingly strong relationship between how a state votes for president and how it votes for Senate over the past few election cycles. And, indeed, Trump’s tumble has coincided with worsening GOP numbers in key states. It may cost the party the Senate.

Democrats need to gain a net of four or five seats to win control of the Senate, depending on whether Hillary Clinton or Trump wins the presidency. Before the conventions, polling in the 10 states whose Senate seats were most likely to flip between parties this November showed a pretty close race. Democratic candidates led in Illinois and Wisconsin, both of which would be pickups for their party. The Republican candidate was leading in Nevada (a seat that Democrats currently control). I didn’t include Indiana in my pre-convention analysis because of Democrat Evan Bayh’s late entrance into the race — we had just one partisan poll that included Bayh — but Democratic chances seemed good there (it would be another Democratic pickup). And Republicans led in the other competitive Senate races, all seats the GOP currently holds, so Democrats looked like they could pick up a net of two seats if everything stayed as it was and the polling leader in each state went on to win.

Since the conventions, however, Trump’s polling has worsened — overall and in states with key Senate races. In the eight states with competitive Senate races and both pre- and post-conventions polling, Trump had previously been down an average of about 6 percentage points; he’s now down an average of 9 points. And while Republican Senate candidates had been up by an average of a little more than 1 percentage point before the conventions in these eight states, they are now down by a little more than 1 point. That is, Republican Senate candidates in key states are still running ahead of Trump, but that cushion may no longer be enough to win now that Trump’s fortunes have worsened.

AUG. 3 MARGIN AUG. 15 MARGIN CHANGE STATE TRUMP GOP SENATE CANDIDATE TRUMP GOP SENATE CANDIDATE TRUMP GOP SENATE CANDIDATE N.H. -2 +2 -11 -5 -9 -7 Illinois -18 -1 -20 -7 -2 -6 Pennsylvania -6 +2 -10 -2 -4 -4 N.C. -1 +4 -5 +1 -4 -3 Wisconsin -9 -8 -12 -11 -3 -3 Nevada -3 +3 -4 +2 -1 -1 Florida -4 +5 -5 +5 -1 0 Ohio -4 +4 -6 +6 -2 +2 Average -3.3 -2.8 Trump may be dragging down Republican Senate candidates Note: The Aug. 15 numbers are as of 7 p.m.

Six of the eight Republican candidates for Senate are polling worse than they were before the conventions. Nothing has changed in Florida, according to the polls. And Sen. Rob Portman in Ohio is the only Republican whose fortunes have improved. (That may be partially because he has a massive fundraising edge over his Democratic opponent, Ted Strickland.) The biggest shifts have been in Illinois, New Hampshire and Pennsylvania, and in the latter two, the leader flipped.

Among the eight states, the most precipitous drop for both Trump and the GOP Senate candidate happened in New Hampshire, where Republican Sen. Kelly Ayotte had led in most polls before the conventions. Since then, she has trailed in all four polls of the state that have been released. MassINC pollster Steve Koczela, who conducted one of the surveys in the New Hampshire average, had told me that Ayotte’s troubles are at least partially because of “how closely tied the Ayotte and Trump vote are” and that he saw that “as evidence that Trump is hurting her.”

Republicans have also seen their prospects worsen in Pennsylvania. Trump is now down 10 percentage points in the state, a headwind that may be too much for Republican Sen. Pat Toomey to overcome. Toomey, like Ayotte, had been leading in most polls before the conventions. But he has trailed in four of the five polls conducted since the conventions. Toomey’s slide, in particular, should worry Republicans. He has made it clear that he is not a Trump fan and has avoided appearing with Trump when he visits the Keystone State. And yet, their fates still seem tied. It may be that down-ballot Republicans can only do so much to keep themselves from getting swept up in an anti-Trump tide.

Democrats now lead in enough states to take back the Senate — so long as Clinton holds on to her large lead. If the favorites in the polls win, the Democrats would flip and pick up the seats in Illinois, Indiana, New Hampshire, Pennsylvania and Wisconsin. Republicans would pick up Nevada and hold onto Florida, North Carolina and Ohio. Of course, many of these races are close, and there’s plenty of time before Election Day. The fight for the Senate isn’t over by a long shot. Republicans and Trump — or Republicans without Trump — could rebound.

On the other hand, Democrats have a lot more pickup opportunities in the Senate than Republicans do, such as in North Carolina, where the race has been tightening. If Trump’s numbers fall even further or Republican Senate candidates fade down the stretch, as some did in 2012, the Senate picture may become even gloomier for the GOP.