Washington (CNN) As the impeachment trial of President Donald Trump gets underway, Mitch McConnell has to pull off a political balancing act. The Senate Majority Leader must weigh the desires of moderate Republicans who want him to allow witnesses and documents, while also delivering a quick and clean acquittal of the President.

But McConnell has a bigger problem to worry about. In less than 10 months, he'll need to ensure Republicans keep control of the Senate on Election Day.

Of the 35 Senate races this year, Republicans have to defend their seats in 23. While most are in solidly red states, there are a handful of Republicans running for reelection in blue states, like Maine, as well as in swing states that have been trending away from Trump's GOP, including Colorado, Arizona and North Carolina.

Add to that an open GOP seat in Kansas that is now on the table for Democrats after McConnell's unsuccessful attempt to lure Secretary of State Mike Pompeo to run, and it's enough to make Republicans nervous about keeping their four-seat majority.

Interviews with half a dozen GOP operatives over the past six weeks reveal some growing apprehension, not so much about McConnell's abilities as a political mastermind -- or even about impeachment. In fact, Trump's approval rating has ticked up since Democrats opened the impeachment investigation.

Rather, there are deeper concerns. Coming off their wins in the 2018 midterms, Democrats and liberal interest groups are carrying momentum into 2020 -- they have a more engaged voter base, they're running against an historically unpopular President -- and for the second cycle in a row -- they could have more money to spend than the GOP.

"We're on defense," McConnell said on a recent episode of the National Republican Senatorial Committee's weekly podcast. "We have three or four (races) that we know are going to be knock-down, drag-out fights.

That's a clarion call to donors that also has the benefit of being true.

Though his majority has been slim, McConnell has wielded his power effectively, blocking legislation from the Democratic House of Representatives and confirming a steady stream of conservative judicial nominations.

If Trump wins reelection, a Republican Senate retains the GOP's advantage in Washington. A Democratic Senate, on the other hand, assuming Democrats keep the House, would stall Trump's second term just as soon as it began. Should a Democrat win the White House, Republicans controlling the Senate would effectively block progressive agenda items being debated on the campaign trail. Should Republicans lose the Senate and the White House, Democrats would have a clear path to raising taxes on the wealthy and increasing the government role in health care.

"For the business community," said Scott Reed, the chamber's senior political strategist, "keeping the Senate is our number one, two and three priority."

The responsibility for pulling that off ultimately falls to McConnell, and it may be the biggest test of his leadership yet.

The McConnell show

Senate Majority Leader Mitch McConnell, Republican of Kentucky, holds a press conference at the US Capitol in Washington, DC, December 17, 2019.

Now in his 13th year as the Senate Republican leader (his fifth as majority leader) and running for a seventh term himself, McConnell is the de facto manager of the Senate GOP's campaign, heading up everything from candidate recruitment to fundraising to strategy.

"This is the McConnell show, absolutely," said one Washington Republican strategist. "He's pulling all the strings here."

McConnell, whose Senate office did not respond to a request for comment, has a physician's approach to keeping the Senate: first, do no harm.

That means ensuring vulnerable GOP senators aren't put in difficult political positions. He's done this in large part by limiting tough votes on legislation -- few of the hundreds of bills passed by the Democratic House have been brought up in the Senate.

But the impeachment trial requires a more precise calibration, particularly when it comes to voting on whether to allow witnesses.

"I believe the vote on witnesses will be more politically perilous for the vulnerable Republican senators than the ultimate vote to acquit or convict," said Paul Begala, a veteran Democratic strategist and a CNN contributor.

Sen. Susan Collins, R-Maine, arrives in the Capitol for a vote on Wednesday, Dec. 11, 2019.

That peril is exemplified by Maine Sen. Susan Collins, who has been given wide berth by McConnell to take a more thoughtful posture on the impeachment trial procedure. Collins has a moderate reputation in a state that frequently rewards independence and bipartisanship. She released a statement last Thursday affirming her support for allowing witnesses in the impeachment trial -- even as every Republican strategist who spoke to CNN considers it unlikely she ends up voting to convict Trump.

"Unity is the key," said the GOP strategist. "You go to the path of least resistance, and tell the rest of the conference, 'We're not going to give Susan Collins s**t because she's expressing openness to certain things.'"

McConnell's plan has to be flexible enough to account for Trump's divergence of support across the country, allowing candidates to embrace him where he's popular and ignore him where he's not.

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That calculation explains the difference in approach between Sen. Martha McSally of Arizona and Sen. Cory Gardner of Colorado. While McSally has explicitly courted the Trump base , Gardner has been practically invisible during the run-up to the impeachment trial.

Whatever the difference in strategy, one thing is clear: no vulnerable Republican can afford to criticize Trump.

"The threat of crossing Trump and going against him weighs more heavily on the minds of these senators than reaching to the middle," said Jessica Taylor, the Senate and governors editor at the nonpartisan Cook Political Report. "Republicans cannot afford to depress Republican enthusiasm by crossing President Trump."

Team Mitch

The job of coordinating this plan falls to a loose network of strategists, operatives and organizations, many of whom have strong ties to McConnell.

Chief among them, say two GOP strategists who spoke to CNN, is Cavalry LLC, a political consulting firm founded by two former McConnell aides, Josh Holmes and John Ashbrook. It was Cavalry that co-opted the epithet "Cocaine Mitch," a moniker initially meant to tie McConnell to a drug bust on a ship owned by the family of his wife, Transportation Secretary Elaine Chao. McConnell's campaign, which Cavalry advises, put the nickname on T-shirts to sell online.

The effort also includes the NRSC as well as McConnell's super PAC, the Senate Leadership Fund, which is helmed by Mike Duncan, the former RNC chairman and a Kentuckian, and Steven Law, McConnell's former chief of staff.

Created in 2015 to help protect the GOP Senate majority, SLF is now in its third election cycle and is expected to continue to play a supportive role. Jack Pandol, the communications director at SLF, told CNN the super PAC's goal is to raise $200 million for the 2020 cycle -- far larger than the group's hauls in either of the past two cycles.

The brightest spot for Republicans in 2019 has been with the NRSC, which has seen the largest fundraising haul ever for either party's Senate campaign committee in an off year: $70 million. That's a helpful start for GOP campaigns in what is already shaping up to be the most expensive Senate cycle ever.

According to data from the Campaign Media Analysis Group,almost $50 million has been spent on Senate races this cycle through mid-January -- more than two-thirds of which has been directed to the four key battlegrounds of North Carolina, Maine, Arizona and Colorado.

So far, most of that money is being spent by Democratic campaigns and progressive groups. To counteract that, the political arm of the US Chamber of Commerce jumped in last fall with the earliest ad buys it's ever made in a Senate cycle.

NRA executive Wayne LaPierre arrives prior to a speech by President Trump at the NRA Annual Meeting in Indianapolis on April 26, 2019.

That could be a sign of desperation by Republican-friendly outside organizations, given that one of the party's biggest boosters, the National Rifle Association, is severely handicapped this cycle. The group's political operation has collapsed over the past year amid internal disputes and lawsuits.

Some Republicans hope the President's reelection operation can supplement what the NRA has been able to deliver, but others told CNN the gun-rights lobby may be irreplaceable this cycle.

"There haven't been any groups to fill the void," said one Republican strategist.

The four most critical races

Three of the most vulnerable senators in 2020 are Susan Collins of Maine (left), Cory Gardner of Colorado (center) and Martha McSally of Arizona (right).

In the end, the fight over the majority will all come down to Republicans' ability to defend four seats.

Gardner is by far the most endangered, given the state's Democratic tilt in recent years, particularly in presidential elections. (George W. Bush was the last Republican to win the state, in 2004.) The likely Democratic nominee is former presidential candidate John Hickenlooper, who was elected governor twice and remains one of the state's most popular politicians.

Trump remains underwater with voters in Colorado, and Gardner's task of uniting rural Trump voters and suburbanites remains perhaps the toughest of any GOP incumbent.

In neighboring Arizona, McSally occupies an odd position. The former House member and Air Force fighter pilot ran for her state's other Senate seat in 2018 and lost to Democrat Kyrsten Sinema. McSally was then appointed to the Senate seat previously held by the late John McCain and held throughout 2018 by Jon Kyl, who resigned at the end of the year to make way for McSally.

So 2020 will be McSally's second chance in two years to face Arizona voters -- and she's hoping to benefit from incumbency and a shakeup of her campaign team since her 2018 defeat. One GOP strategist told CNN the Republican should run in 2020 "like she's running for sheriff of Maricopa County" -- that is, the Phoenix suburbs.

Martha McSally (R-AZ) conducts a hearing at the U.S. Capitol May 23, 2017 in Washington, DC. The subcommittee heard testimony about visa overstays and the federal government's progress in developing a biometric entrance and exit tracking system for people crossing international borders.

Unfortunately for her, McSally has drawn as a Democratic opponent astronaut Mark Kelly, the husband of former Rep. Gabby Giffords. Kelly has been the best Democratic fundraiser this cycle, ending 2019 with more than $20 million, nearly twice as much as McSally's $12 million. He ended the year with $6.2 million haul in the last quarter, besting the $4 million raised by McSally during that time.

In Maine, Collins remains a top target for progressives motivated by her vote to confirm Supreme Court Justice Brett Kavanaugh in 2018. The expected ad-spending war of attrition in Maine guarantees the race between Collins and her likely Democratic opponent, Speaker of the state House Sara Gideon, will be competitive.

While Maine consistently votes for Democrats at the presidential level, Trump lost the state in 2016 by just three points and won the vote in one of its congressional districts, giving him a single electoral vote. Collins, meanwhile, has had four terms in the Senate to carve out a moderate and independent reputation that Maine often rewards at the ballot box. Of all the endangered Republicans, Collins is viewed as the best positioned to win over split-ticket voters in her state, thanks to this reputation.

Another target for Democrats is Thom Tillis in North Carolina, who six years ago beat incumbent Democrat Kay Hagan by just 1.5 points. While 2008 was the only time this century a Democratic presidential candidate has won the Tarheel State, it remains a swing state. Trump won it by just three points in 2016, the same year a Democrat, Roy Cooper, narrowly won the governor's race.

Tillis has already fended off a GOP primary challenger from the right, a self-funding candidate who dropped out in late 2019. He's also benefiting from a competitive Democratic primary between the establishment-backed lawyer Cal Cunningham and a progressive favorite, state Sen. Erica Smith. Along with Arizona, Republican strategists agree, North Carolina will most closely track with the presidential results there.

In all four races, the Republican campaigns are taking their cues from McConnell and the strategy-makers from Washington.

"If it's going to be a national election, then you need to take cues from the national boss," said the Washington GOP strategist. "At the end of the day, this is going to be a polarized election."