Hillary Clinton's increasingly dominant lead in the presidential race is solidifying many Republicans' worst 2016 fears that Donald Trump will cost the party not only the White House but also control of the Senate.
"The bottom is starting to fall out a little earlier than expected," says a top Senate GOP campaign aide who requested anonymity to speak candidly about the state of the race. "We started off with a very difficult map. No matter what, this was going to be a very difficult year."
The aide says Trump's ailing campaign is an additional drag on the Senate battlefield. The end result, the aide concedes, is a likely Democratic takeover this November.
That candor is widely — if still privately — shared by increasing numbers of Senate GOP campaign operatives who believe that Trump is destined to lose the presidential race and that the Republican Party's short-lived, two-year majority will go with it.
Republicans took control of the Senate in the 2014 midterm elections and enjoy a 54-46 majority (there are two independents but they caucus with Democrats). Democrats need just four seats for a takeover if Clinton wins the White House because the vice president is the tiebreaker in an evenly divided Senate.
Democrats are already heavily favored to pick up two GOP-held seats in Illinois and Wisconsin, leaving them with just two additional wins necessary for a takeover. The two parties are seriously contesting races in 10 additional states, including races that overlap with the presidential battlegrounds of Pennsylvania, Ohio, North Carolina and New Hampshire.
Senate operatives are particularly dismayed by Trump's post-convention slump — both in polling and in performance — which the latest polls indicate is negatively affecting Republicans down the ballot.
As FiveThirtyEight noted on Tuesday, six of the eight Republicans running in top-tier Senate races are polling worse since the conventions. Only two Republican incumbents, Ohio Sen. Rob Portman and Florida Sen. Marco Rubio, have either maintained or advanced their standings since then.
Clinton is also expanding the map into states once thought of as safe territory for Republicans. In North Carolina, Clinton's recent surge is seriously imperiling two-term GOP Sen. Richard Burr, who recently fell behind little-known Democratic challenger Deborah Ross, 46 percent to 44 percent, in the latest Marist poll.
In Georgia, GOP Sen. Johnny Isakson went on the air this week with his first campaign ad of the cycle. The spot, featuring an older woman who is a lifelong Democrat, is an obvious bid to appeal to a key demographic that Trump has alienated.
Republican and Democratic campaign operatives also agree that races in GOP-held Missouri and Indiana will tighten as Election Day approaches.
From the outset, Republicans had only two opportunities to pick up a Democratic-held seat in 2016. But in Colorado, Republicans nominated a conservative candidate who is polling so far behind Democratic incumbent Sen. Michael Bennet that Bennet is cruising toward re-election.
The lone opportunity for a Republican pickup is Nevada, where Clinton's edge is narrower over Trump and the Senate race remains competitive for both the presidential race and the seat held by retiring Senate Minority Leader Harry Reid. "We have to win Nevada to have any hope of holding the Senate," the GOP aide says.
Will more voters split their tickets this fall?
For Senate GOP candidates to win in a state carried by Clinton this November, they will need to directly appeal to Clinton voters who are willing to split their ticket between parties.
Professor David Kimball with the University of Missouri, St. Louis has studied split-ticket voting patterns and is skeptical that a critical mass of voters will split their tickets in enough states to deliver both a Clinton victory and a Senate GOP majority on Election Day.
"Most voters simply intend to be, No. 1, in support of their party and thus aren't receptive to strategic or nuanced arguments," Kimball says. "And if they are going to split their ticket it's going to be for a candidate that is personally appealing to them, or to avoid a candidate that's personally unappealing."
Senate candidates who have successfully outperformed the top of the ticket have generally fallen into two camps. The first are personally popular incumbents like Democratic Sen. Joe Manchin of West Virginia or GOP Sen. Susan Collins of Maine, who have their own distinct brands and regularly outperform their party.
Other senators who have succeeded against the top of the ticket faced fatally flawed opponents, as Sen. Claire McCaskill, D-Mo., did in 2012 against Republican Todd Akin, who made controversial statements about rape and pregnancy.
Many of this year's GOP incumbents are first-term senators like Pennsylvania's Pat Toomey and New Hampshire's Kelly Ayotte. Campaign operatives praise both for running strong campaigns, but there is deep skepticism that either can win if Clinton decisively wins their states.
Trump's missing ground game
Nonpartisan election analyst Sean Trende of RealClearPolitics.com says Republicans are further damaged by the lack of any organizational heft at the top of the ticket.
"A more serious problem for Republicans is the lack of a Trump campaign," Trende says, noting that presidential candidates tend to drive voter identification, mobilization and turnout that benefit candidates down the ballot. The Trump campaign is not operating any semblance of a traditional campaign operation to get out the vote in critical states. "Hillary Clinton is drumming up support and a lot of these Senate candidates are on their own," he says.
Trende forecasts a Democratic takeover, although he notes that down-ballot races generally don't cement themselves until the closing weeks of the election.
Trende forecasts Democratic pickups in Illinois and Wisconsin and likely in Indiana, where former Democratic Sen. Evan Bayh is attempting a comeback and is currently favored to win against GOP Rep. Todd Young.
That would put Democrats just one seat away from a takeover. "It basically comes down to: Can Democrats hold Nevada, and win one race in Pennsylvania, New Hampshire, Florida, Ohio or some other state?" Trende says.
In the coming weeks, it may no longer be a question of whether Democrats take over the Senate, but rather how big of a majority they will have come Election Day.