The Republican Party may lose its majority in the US Senate, senior congressional aides warn, with one blaming Donald Trump as a drag on Republican candidates.
Several Senate aides from both parties privately warned of trouble for Republicans in the 8 November elections.
"Things are not good ... the Senate is gone," said one Republican aide who asked not to be identified in order to candidly discuss the turbulent outlook for the 2016 campaign.
Citing polling, the aide said Republicans could lose Senate seats in six battleground states: Pennsylvania, Illinois, Wisconsin, New Hampshire, North Carolina and Missouri.
The party has had a majority for two years, holding 54 of the Senate's 100 seats.
Democrats must snatch four seats to win a majority, provided their presidential candidate Hillary Clinton beats Mr Trump. That would make Mrs Clinton's running mate, Tim Kaine, the tie-breaking Senate vote since the vice president votes in order to break a tie.
On Tuesday, the non-partisan Cook Political Report predicted Democrats would gain five to seven seats. Such a result would leave them short of the 60 votes needed to easily get things done in the Senate, but it would provide a majority.
A less pessimistic Senate Republican aide said Senate control still "could go either way" but sketched out problems.
In Pennsylvania, the aide said, Senator Pat Toomey has to "fight off dead weight at the top of the ticket," referring to Trump.
Nationally, "the reason we don't hold the Senate, if we don't, is because of Donald Trump," the aide said.
In a volatile and unpredictable year there is still time for trends to reverse and Democrats stressed they were not breaking out the champagne.
According to RealClear Politics, Repblican candidates are leading in Pennsylvania and Missouri and North Carolina.
Still, a senior Senate Democratic aide said, "We have a lot more paths to a majority than they do."
Republicans entered the 2016 Senate races at a disadvantage, having to defend 24 seats to only 10 for Democrats.
Democratic President Barack Obama, who is still relatively popular, won at least one of his two White House races in most of the states where Republicans currently are struggling, such as New Hampshire, Pennsylvania and Nevada.
Mrs Clinton leads Mr Trump by about five points in some national polls. Studies show a pattern of voting in recent decades in which the outcome of Senate races has had a significant correlation to who wins the White House.
Polling in the hardest-fought states points to extremely close races with Republican incumbents' backs against the wall.
In New Hampshire, Republican incumbent Kelly Ayotte is in a tough race with Democratic challenger Maggie Hassan.
University of New Hampshire political science professor Dante Scala said Republican senators in that state and Pennsylvania face the "one-two punch" of independent voters potentially being turned off by Mr Trump and an electorate that leans Democratic in presidential election years like 2016. "Given the overall environment, it tilts to Hassan," Prof Scala said.