First, if Trump’s poll numbers slide enough to convince supporters that defeat is inevitable, they may opt to stay home. That would cost down-ballot Republicans as well.

Second, Republican officeholders who criticize Trump may alienate his supporters, who may turn out for the Republican presidential nominee but bypass House and Senate races.

Third, large Clinton victories (instead of narrow wins) in competitive states would force Republican Senate campaigns to get more voters to split their tickets. That is an additional burden that GOP political operatives would much prefer to avoid.

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And fourth, Republican Senate hopefuls will continue to be peppered with questions about whether they agree or disagree with Trump, making it more difficult for them to localize their races and deliver winning messages. That is always a problem, as Republican Sen. Kelly Ayotte (R-N.H.) discovered after she unwisely said during a debate that Trump is a role model.

So where do the developments of the last week leave the fight for the Senate?

Two Republican seats, Illinois and Wisconsin, are already poised to flip. Trump’s weakened position has undermined GOP prospects in Pennsylvania and New Hampshire, both of which Clinton should win. Republican Sens. Pat Toomey (Pa.) and Ayotte must now run even farther ahead of the top of their tickets.

Trump is also an additional problem for Sen. Richard Burr (R-N.C.), who is now running no better than even against Democrat Deborah Ross, a relatively weak challenger with plenty of baggage.

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Burr was slow to attack his opponent, and his task is complicated by the fact that Clinton is more likely than not to carry the Tar Heel State. The state’s Republican governor, who is seeking reelection, may well be headed for defeat, and the overall political environment in the state is challenging for Republicans.

Missouri was not expected to be a problem for the GOP, but Republican Sen. Roy Blunt is in an unexpected dogfight against Democratic challenger Jason Kander. Trump is underperforming in the Show Me State, as he is elsewhere, but the state’s fundamentals are favorable for Blunt.

One race has moved in the opposite direction: Indiana. Former senator Evan Bayh (D), who began his comeback bid with high favorables and a huge lead over Republican Todd Young, is under attack now for job hunting – including meeting with potential employers in his Capitol Hill office while still a senator.

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But Bayh’s other problems, from his lack of time spent recently in Indiana to some of his Senate votes, had already done damage to his image. This race is now a toss-up, something Democrats did not expect.

In Nevada, the one competitive race currently held by a Democrat, multiple polls have shown Republican Joe Heck with a narrow lead. But given uncertainties about Latino turnout and Heck’s rejection of Trump, this race is too-close-to-call.

Republican Senate candidates initially succeeded in separating themselves from Trump because his political problems stem so heavily from his personal flaws and atypical behavior. Unlike Barry Goldwater (in 1964) and George McGovern (in 1972), who helped create partisan political waves for the opposition because they were perceived as extreme ideological nominees, voters are not yet painting Trump and down-ballot Republicans with the same broad brush.

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But one veteran Republican observer who requested anonymity notes that Trump’s slide in the polls has opened up a large gap in key Senate races between the percentage of the vote that Trump is getting and the percentage that the Republican Senate nominee is getting.

“As we approach the election, that gap could close – and not to the benefit of Republican Senate candidates,” worried the Republican who cited turnout, state voting patterns and Trump’s weakness as problems for GOP Senate nominees in tight races.

Before the Trump tape issue emerged, there was no indication that a national partisan wave had developed. That’s important, because in a partisan wave Senate seats often fall in one direction. However, the lack of a wave in no way guarantees the two parties will “split” toss-up races, and Trump’s candidacy is so damaged that experienced Republican observers are extremely nervous.

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But while the Trump campaign’s implosion obviously ratchets up the risk for the GOP, it may not be fatal. Trump’s supporters are intensely loyal and energized, and Clinton’s personal ratings remain poor. Her weakness gives Republican candidates an opportunity to attract voters who dislike both presidential nominees by returning to their “don’t give Hillary Clinton a blank check” message.

That message, which disappeared as the presidential race appeared to tighten during September, should be back in full force in swing states during the final weeks of the campaign.

Unlike the presidential contest, which has had a clear trajectory since after the national party conventions, the fight for the Senate has always been murkier.

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A little more than a month ago I wrote that Republican Senate prospects had improved compared to six months earlier, primarily because Republican incumbents in Ohio and Florida were in fundamentally better shape than expected. But, I also concluded, “Right now, the most likely outcome is a Democratic gain of four or five seats.”

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The strangest presidential campaign in American political history could well get stranger before Nov. 8. But right now, Democratic Senate gains in the four to five0seat range still look most likely – which would give Democrats control of the Senate.

Significantly larger Democratic gains certainly are possible, of course, especially if GOP-held Senate seats in Florida and Ohio once again become at risk.