A still from an ad by the Democratic Senate Majority PAC tying Sen. Kelly Ayotte (R-N.H.) to Donald Trump. (Via YouTube)

A big reason national Republicans are begrudgingly supporting Donald Trump's 2016 campaign is because they recognize their fortunes are tied to it. If Trump implodes, the logic goes, so too does the party — in this election and perhaps in elections to come.

And that's especially the case in the Senate, where the GOP's majority was endangered even before Trump came along. Republicans face a very tough map, defending eight seats in states that President Obama won in either 2008 or 2012. Democrats need to win just four of them to take control of the chamber.

The good news for Republicans: Many of their candidates are outperforming the top of the ticket. The bad news? That still might not be enough — especially if things are as bad for Trump as they look right now.

Case in point: A WBUR/MassInc poll in New Hampshire shows Trump losing that swing state by a stunning 15 points — his biggest deficit yet. And likewise, it shows Sen. Kelly Ayotte (R-N.H.) trailing by 10 points — her biggest deficit yet.

Meanwhile, a Franklin & Marshall College poll in Pennsylvania shows Trump trailing there by 13 points among registered voters — his biggest deficit since April. And likewise, Sen. Patrick J. Toomey (R-Pa.) faces his biggest deficit yet among registered voters — eight points.

A few caveats: Toomey is much closer among likely voters, trailing just 39-38. Also, these two are the only high-quality Senate polls we have since the Democratic and Republican conventions late last month, so it's still very early, and we could still be seeing the remnants of the bump Democrats got from their superior convention.

But these two polls are instructive, and the seats also happen to be very important ones. That's because they are big parts of the Republican Senate firewall.

Republicans will have a very tough time defending both Sens. Mark Kirk in Illinois and Ron Johnson in Wisconsin. If those seats are gone, New Hampshire and Pennsylvania could be very decisive states. We even have them as Democrats' third- and fifth-best pickup opportunities on our latest list of the 10 Senate races most likely to change hands. (Others in that firewall include Florida and Ohio, along with the newly competitive Indiana Senate race featuring former Democratic senator Evan Bayh.)

The fact that these polls show Ayotte's and Toomey's fortunes falling alongside Trump's reinforces just how important the top of the ticket is. Both led their races for months in early polling, but with Trump now down by double-digits in their states, they trail in most recent polls. Even as neither Ayotte nor her Democratic opponent, Gov. Maggie Hassan, have seen big shifts in their image ratings, Ayotte suddenly trails big in this poll.

Of course, whenever one party or the other faces a tough national environment, their leaders will insist that the next election will be localized and that candidates matter. "All politics is local," the famous Tip O'Neill saying goes. But that's increasingly not the case in federal elections, with fewer and fewer voters splitting their tickets by voting for one party for president and the other for Congress.

And polling so far in 2016 suggests there's only so much distance Republicans can put between themselves and Trump.

The five-point difference between Ayotte's deficit in the poll (10 points) and Trump's (15) is actually the most she's outperformed Trump by in recent months. Previous polls from WMUR and Democratic-leaning automated pollster Public Policy Polling have showed Trump trailing Hillary Clinton and Ayotte trailing Hassan by the low single digits.

In Pennsylvania, Toomey has outpolled the top of his ticket for months and looked like a strong bet for reelection as Democrats struggled to recruit, eventually settling on Katie McGinty. But even he can apparently only separate himself from Trump's fate so much. A recent Public Policy Polling poll showed Trump down 4 and Toomey up 1. A Suffolk poll showed Trump down 9 points and Toomey down 7. An NBC/Marist College poll showed Trump down 9 and Toomey down 3.

As noted above, Toomey does much better when you focus only on likely voters — trailing by just one point even as Trump trails by 11. That's important to keep an eye on as we move forward, and it gets easier to determine who is actually likely to vote.

And two GOP incumbents running in key races in Florida and Ohio — Marco Rubio and Rob Portman — are similarly outperforming Trump. Recent polls in Florida suggests Rubio is running as much as 10 points ahead of Trump's margin; an NBC/Marist poll showed him up 3 points even as Trump trailed by 7. Most recent polls have Ohio as a tie at the presidential level, but Portman has led by as much as 5 or 7 points.

Update: And now a new Suffolk poll shows Rubio running well ahead of Trump in Florida. While Trump is down by 6, Rubio is up by 13. It should be noted, though, that Rubio's likely opponent, Rep. Patrick Murphy (D), has faced some very tough headlines in recent weeks.

Those differences are good for Republicans, and they'll need to keep it up. But if Trump is trailing in swing states by double digits or even the high single digits, it's going to be very tough for candidates such as Ayotte, Portman and Toomey to stick around.