Washington (CNN) Republican leaders are watching Donald Trump's campaign with growing alarm as they fear a landslide at the top of the ticket could wipe away their hard-fought congressional majority.

For months, top GOP leaders had counseled their candidates to run their own races, separate themselves from the ugly back-and-forth of the presidential contest and focus on the accomplishments Republicans have achieved on Capitol Hill for their home states.

But in interviews with top Republicans in Washington, several privately told CNN that there's not much they can do if Trump loses in each of the battleground states by 10 points or more. And they are pushing Trump hard to right the ship -- namely by focusing exclusively on Hillary Clinton and seizing on her vulnerabilities -- to help avoid an electoral bloodbath in the fall.

"If it's 10 points or more, we are in big trouble," said one top Republican, who asked not to be named to talk candidly about the Senate landscape.

Republicans believe there's still ample time to fix their problems, particularly given Clinton's high negative ratings in many polls. And they are confident in the strength of their Senate GOP incumbents, many of whom are well-funded and have built robust field operations that could help them outperform Trump in their states.

But in an interview at the Republican National Convention last month, Senate Majority Leader Mitch McConnell said flatly, "I don't think we know yet" whether Trump will help or hurt Republicans down-ticket. He said each of the races are "big, well-funded, individual pictures" not necessarily tied to Trump.

"I think we'll have to see where the presidential election ends up," McConnell told CNN. "One of the good things about the Senate is the races are big enough to stand on their own."

Asked if Republicans would keep control of the Senate, McConnell added confidently: "We will."

McCain to Pence: Focus on Clinton

But Senate Republicans also need help from Trump -- namely ensuring he remains competitive until Election Day.

In a private meeting this week at a Phoenix airport, Sen. John McCain of Arizona urged Trump's running mate, Mike Pence, to keep the top of the ticket trained on Clinton -- and not engage in squabbles with other Republicans or distractions like his recent feud with Muslim-American parents whose son was killing while serving in the Iraq War, according to a source familiar with the session. Pence, the source said, agreed.

For Democrats to regain the Senate, they need to pick up four seats if Clinton wins the White House and five if Trump pulls off a victory. And there are ample opportunities for Democrats in blue and swing states -- namely in Pennsylvania, Illinois, Wisconsin, Ohio and New Hampshire.

Other seats on the radar are the ones held by McCain in Arizona, Marco Rubio in Florida and Indiana's Dan Coats, who is vacating his seat.

Democrats are defending just one seat that Republicans stand a serious shot of winning at the moment -- Nevada -- being vacated by Senate Minority Leader Harry Reid. Republicans once were hopeful they had a shot in Colorado, but GOP recruiting failures led to the nomination of conservative Darryl Glenn, who is trailing Democratic Sen. Michael Bennet by double digits in recent polls.

With Trump's standing dropping markedly since the convention, the Democratic strategy is simple: Tie every Republican to their unpopular standard-bearer.

JUST WATCHED Harry Reid talks Trump, down-ballot Senate races Replay More Videos ... MUST WATCH Harry Reid talks Trump, down-ballot Senate races 01:57

Senate Democrats are so deadset on this strategy that Reid publicly rebuked President Barack Obama for saying at the Democratic National Convention that Trump's message "wasn't particularly Republican, and it sure wasn't conservative."

"I would disagree with the President," Reid told CNN. "The reason we have Donald Trump is because what has happened in the Congress of the United States by the Republicans ... they've created Donald Trump."

To help save the Senate, Republicans are banking on the support of well-funded outside groups, including McConnell's leadership PAC and the network of organizations bankrolled by the Koch brothers, who plan to spend $250 million on their political activities this cycle.

"What we've seen is the Republican candidates we're favoring, that we're supporting, have been very durable," said Mark Holden, senior vice president of Koch Industries.

Senate Dems reel in convention bounce

An NBC/Wall Street Journal poll released Friday shows Democrats with an edge nationally on the question of which party should be in charge of Congress -- 47% of registered voters prefer a Democratic-controlled Congress, with 43% favoring a GOP-controlled one.

In a conference call with reporters on Friday, several Senate Democrats sought to connect GOP senators to Trump's latest comments on national security issues -- and urged them to officially withdraw their support.

Trump had his first big primary win in New Hampshire in January, but a new WBUR poll out this week indicates Clinton has a 15-point lead in the state. And in a clear warning sign for the GOP, the same poll shows Sen. Kelly Ayotte trailing Democratic Gov. Maggie Hassan by 10 points.

While both sides believe the New Hampshire Senate race is much tighter, the Trump effect is putting Ayotte in a bind. The first-term senator has indicated she would support Trump -- but has sought to draw a contrast with him by saying she would not endorse him and criticizing several of his more controversial statements.

Ayotte's New Hampshire colleague, Democratic Sen. Jeanne Shaheen, attacked GOP senators for trying to "have it both ways" in backing the nominee while trying to distance themselves from him.

"This idea that 'I support him but I don't endorse him' -- that's baloney. When you support somebody, you effectively endorse them," Shaheen said on a conference call with reporters Friday.

Earlier this week, Trump called Ayotte "weak" and declined to endorse her. Ayotte responded by saying, "I call it like I see it, and I'm always going to stand up for our military families and what's best for the people of New Hampshire."

GOP officials in the state note its reputation for rewarding independent-minded candidates and argue that the episode is more likely to help Ayotte with voters. But given Trump's appeal with GOP voters in the state, Ayotte can't afford to alienate herself from a crucial voting bloc during a close Senate race.

The same holds true in Pennsylvania, a state Trump won overwhelmingly with nearly 57% of the vote in the April GOP primary. In that state. Sen. Pat Toomey has sharply criticized Trump as the former appeals to swing voters -- particularly in the Philadelphia suburbs.

Trump has predicted he would win Pennsylvania, a state that has a high percentage of white voters without advanced degrees, a demographic group that has overwhelmingly favored him. He continues to do well -- 53% to 31% -- with that voting bloc in the state, according to a poll released earlier this week by Franklin and Marshall.

But statewide that same poll shows Clinton with a double-digit lead -- 49%-38% -- over Trump.

The Senate race between Toomey and Katie McGinty, the Democratic challenger who served in the Clinton administration, is much tighter, with McGinty holding a 1-point lead in the same poll.

Toomey told reporters on a conference call Friday that he believed voters in his state would split their ticket in November, saying, that Trump "is in a category unto himself." With respect to his own race, Toomey said voters "will make a completely separate decision about the person they want representing them in the United States Senate," according to press reports.

Toomey continues to say that he is still deciding whether to officially endorse Trump.

In Ohio, polls before both parties' conventions showed the presidential race in a statistical deadheat. Sen. Rob Portman is taking no chances in his race against former Democratic Gov. Ted Strickland, with the Republican's campaign making more than 3 million contacts with voters since last year through phone calls and door knocks, according to Portman's campaign. The idea: Identify swing voters and sell them on Portman's record, regardless of how they feel about the top of the ticket.

In Florida, a new Suffolk poll shows Rubio leading his likely Democratic challenger, Patrick Murphy, by double digits -- and Trump losing to Clinton by just 6 points.

But privately, Republicans acknowledge the fight for Rubio's seat could tighten if Trump's slide continues in Florida.

Both sides believe that Illinois and Wisconsin remain two of the most likely Democratic pickups in the country. In Illinois, GOP Sen. Mark Kirk has revoked his endorsement of Trump, while Wisconsin Sen. Ron Johnson won't give his official backing to the nominee, though he says he'll support him.

"I think there's a huge difference between endorsement and support," Johnson said in a recent CNN interview. "There are obviously disagreements" between himself and Trump, he said.