While Donald Trump is making Republican Party elders nervous amid concerns over his lack of discipline and grassroots infrastructure, another major problem may be brewing -- as polling trends signal the potential damage the campaign could do to Republican congressional candidates and GOP control of Congress.

"Just about everybody has recognized that the Republican Party is deeply divided, and really, it's in a mess," said Larry Sabato of the University of Virginia Center for Politics. "And I think Donald Trump would agree because he's not getting the kind of support that he thinks he deserves."

Sabato cited a range of races where a Trump coattail effect could make a difference.

"Senator Pat Toomey (R-Pa) is very vulnerable, Sabato said.  “Rob Portman (R-Ohio) is less vulnerable; he's doing better than others but he's still nervous about it.  Senator Mark Kirk (R-Il)...  it's hard for anybody to see how he wins."

Sabato also counted among the vulnerable: Senator Ron Johnson (R-Wis), Sen. Marco Rubio (R-Fla.) in Florida, Richard Burr (R-N.C.) and John McCain (R-Az).  And he added this ominous warning: "There could be others popping up on the screen."

Senate Majority Leader Mitch McConnell touched on the issue last week when he told a Louisville, Ky. civic group, "What we're looking for here is a candidate who settles down and follows the script and makes the election about Hillary Clinton."

But it's not just tepid support from party elders  frustrated with Trump's lack of discipline that's raising concern.  It's the Trump campaign's poor grass roots organization, as well as Trump's reliance on social media  at the expense of paid  TV ads.

Swing states that he must win are not polling well in his favor.  A Marist NBC/ Wall Street Journal poll late last week  showed Trump behind Clinton by 5 percent in Florida,  by 9 percent in North Carolina, by 13 percent in Virginia and by 14 percent in Colorado.

One GOP pollster sees a threshold for a down ballot disaster in key swing states. "If he's losing by 4 or 5 points, even 6 points, we can make up that difference," said Ed Goeas. "It's when it starts inching up towards 8, 9, 10 percent difference that you can't make up the difference."

Sabato agreed. "If you're in a competitive race, you're going down. You're not going to be able to survive a 10 percent margin."

Still, Trump continues to pull in big crowds and Nov. 8 remains a long way off. Unpredictable events can change the race. "If he's at the top of his bandwith and she's at the bottom, he can win this election,"  Goeas said.

But that's contingent on Trump doing something that he's not been able to do yet - stay on message in attacking Clinton.  The good news for Trump is that she ranks second to last in favorability of any presidential candidate in history. The bad news for Trump - he ranks dead last in the same category.

Doug McKelway joined Fox News Channel (FNC) in November 2010 and serves as a Washington-based correspondent. Click here for more information on Doug McKelway