The Senate Leadership Fund (SLF) on Tuesday announced its first major ad buy of the 2018 cycle — a $6.4 million blitz backing Republican candidates, signaling growing GOP concerns about protecting their Senate majority in November.

Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellPelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Senate GOP aims to confirm Trump court pick by Oct. 29: report Trump argues full Supreme Court needed to settle potential election disputes MORE (R-Ky.) warned that same day of a “storm” facing Senate Republican candidates and acknowledged that Sen. Ted Cruz Rafael (Ted) Edward CruzTrump argues full Supreme Court needed to settle potential election disputes Press: Notorious RBG vs Notorious GOP The Hill's Morning Report - Sponsored by Facebook - Washington on edge amid SCOTUS vacancy MORE (R) could lose in Texas, a GOP stronghold for decades.

New polls show Sen. Joe Donnelly Joseph (Joe) Simon DonnellyTrump meets with potential Supreme Court pick Amy Coney Barrett at White House Names to watch as Trump picks Ginsburg replacement on Supreme Court Momentum growing among Republicans for Supreme Court vote before Election Day MORE (D-Ind.) and former Tennessee Gov. Phil Bredesen (D) leading their GOP opponents in Indiana and Tennessee, two strongly pro-Trump states, and Cruz in a dead heat with Democratic challenger Rep. Beto O’Rourke.

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The recent surveys are disappointing to Republicans who had hoped that Tennessee would be an easy victory. Shortly before Labor Day, a senior Senate Republican aide dismissed Bredesen’s prospects as “hype” and predicted the party could pick up as many as four Senate seats in the midterm elections.

Republican strategists have also counted Donnelly’s seat in Indiana as a likely pickup and were surprised by an NBC/Marist poll released last week that showed him up 6 points against Republican businessman Mike Braun.

“Everybody feels like we’re going to win that, but I thought it would be a better number,” said Chip Saltsman, a GOP strategist. “That’s one I would put on there that surprised me a little bit.”

A second Republican strategist said, “Braun does seem to have let his foot off the gas a little after the primary.”

The strategists attribute Donnelly’s lead to Braun’s lack of name recognition, while pointing out that former Sen. Evan Bayh (D-Ind.) lost a big lead in the 2016 Indiana Senate race. Still, they acknowledge that Donnelly’s advantage in the polls is concerning, especially since it comes just two months before Election Day.

Last week’s NBC/Marist poll also showed that 29 percent of registered voters in the state were unsure or hadn’t heard of Braun.

Tuesday’s announcement by the SLF included $1.4 million aimed at boosting Braun’s name and painting Donnelly as a politician who says one thing at home and votes another way in Washington.

The ad features President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE standing alongside Braun at a boisterous rally declaring: “We want people like Mike Braun.”

The SLF, whose president and CEO, Steven Law, is McConnell’s former chief of staff, also unveiled a $1.1 million ad buy slamming Bredesen for raising taxes and fees by almost $1 billion while serving as governor.

“Phil Bredesen is out of touch,” the ad says.

The Bredesen campaign fired back by noting that he opposed a proposed state income tax and increase in the sales tax and that his efforts to close corporate tax loopholes had strong bipartisan support.

The SLF’s announcement also included a $1.8 million ad buy against Sen. Claire McCaskill Claire Conner McCaskillMomentum growing among Republicans for Supreme Court vote before Election Day Democratic-linked group runs ads in Kansas GOP Senate primary Trump mocked for low attendance at rally MORE (D-Mo.), a $1 million buy against Rep. Jacky Rosen Jacklyn (Jacky) Sheryl RosenHillicon Valley: Election officials prepare for new Russian interference battle | 'Markeyverse' of online fans helps take down a Kennedy | GOP senators unveil bill to update tech liability protections Google, Apple, eBay to meet virtually with lawmakers for tech group's annual fly-in Senate Democrats demand answers on migrant child trafficking during pandemic MORE (D), who is running for Senate in Nevada, and a $350,000 blitz against Sen. Heidi Heitkamp Mary (Heidi) Kathryn HeitkampCentrists, progressives rally around Harris pick for VP 70 former senators propose bipartisan caucus for incumbents Susan Collins set to play pivotal role in impeachment drama MORE (D-N.D.).

Additionally, the group expanded the media buy it launched last week against Sen. Joe Manchin Joseph (Joe) ManchinThe Hill's Morning Report - Sponsored by Facebook - Trump, GOP allies prepare for SCOTUS nomination this week Trump meets with potential Supreme Court pick Amy Coney Barrett at White House Names to watch as Trump picks Ginsburg replacement on Supreme Court MORE (D) in West Virginia by $800,000.

The SLF has spent $13 million so far this cycle, according to the Center for Responsive Politics.

Saltsman, the GOP strategist, said outside groups such as the SLF will be crucial to protecting the Senate Republican majority by bringing down the favorable numbers of Democratic incumbents.

“The path to victory relies on these outside groups laying the wood on candidates like Bredesen,” he said. “It’s the difference between winning and losing. The third-party groups, especially the ones that have done a really good job like Mitch McConnell’s group, they know how important they are.”

Republicans still think they are likely to retain their majority given that the Senate battlegrounds are in pro-Trump states, but they say GOP candidates and outside groups will have to step up their efforts to make up for what McConnell described Tuesday as the “wind” that “is going to be in our face.”

McConnell acknowledged to reporters in Louisville, Ky., that Senate Republicans face a “very challenging election” because their party controls the White House.

“You can’t repeal history, and almost every election two years into any new administration the party of the presidency loses seats,” he said when asked about the chances of keeping the Senate GOP majority. “They don’t always lose the body, but almost always loses seats. And so, we know that this is going to be a very challenging election on the Senate side.”

The political environment has become more challenging for Republicans in recent weeks because of Trump’s dropping poll numbers after what many Republicans admitted was a brutal summer.

The president’s performance at the Helsinki summit with Russian President Vladimir Putin in July received negative reviews, even from fellow Republicans, and his longtime lawyer Michael Cohen last month accused him of directing illegal payments over affairs with two women.

Trump has seen his approval ratings drop in eight regular polls, including surveys conducted by CNN, Gallup, ABC News/Washington Post and Quinnipiac University.

Senate Republicans felt confident about expanding their majority at the start of the election cycle because 10 Democrats face reelection in states carried by Trump in 2016 while only one Republican incumbent, Sen. Dean Heller Dean Arthur HellerOn The Trail: Democrats plan to hammer Trump on Social Security, Medicare Lobbying World Democrats spend big to put Senate in play MORE (Nev.), is running in a state carried by Democratic presidential nominee Hillary Clinton Hillary Diane Rodham ClintonButtigieg stands in as Pence for Harris's debate practice Senate GOP sees early Supreme Court vote as political booster shot Poll: 51 percent of voters want to abolish the electoral college MORE.

Overall, Democrats have to defend 24 incumbents while Republicans have to protect nine.

But the political climate has turned negative for Republicans in recent weeks, despite the booming economy, because Democratic voter enthusiasm is higher than it has been in years and the GOP no longer can run against their favorite foils, Clinton and former President Obama.

One Republican strategist close to McConnell said the GOP leader was sounding the alarm to party faithful not to take anything for granted.

“The message is maps don’t make majorities. The message is the map is really good for Senate Republicans but that doesn’t mean anything if you don’t get out and spend as much as you can and work as hard as you can,” said Scott Jennings.

“There is a historical headwind out there and nothing can be taken for granted. You can’t assume that just because the map is good, you’re going to hold [the majority] no matter what,” he said.

The SLF is hitting McCaskill over immigration with a new television ad juxtaposing a statement McCaskill made in 2006 opposing amnesty for immigrants in the country illegally to her 2013 vote in favor of a bipartisan immigration bill designed to create a pathway to citizenship for an estimated 11 million people.

“In Washington, McCaskill joined the liberals for giving amnesty to 11 million illegal immigrants without border protections in place,” the ad charges. “After 12 years in the Senate, McCaskill’s gone Washington and left Missouri behind.”

The bill also had support from Republicans such as Sens. Jeff Flake Jeffrey (Jeff) Lane FlakeJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Republican former Michigan governor says he's voting for Biden Maybe they just don't like cowboys: The president is successful, some just don't like his style MORE (Ariz.), Marco Rubio Marco Antonio RubioOvernight Defense: Pentagon redirects pandemic funding to defense contractors | US planning for full Afghanistan withdrawal by May | Anti-Trump GOP group puts ads in military papers Democrats step up hardball tactics as Supreme Court fight heats up Press: Notorious RBG vs Notorious GOP MORE (Fla.) and Lindsey Graham Lindsey Olin GrahamSenate GOP aims to confirm Trump court pick by Oct. 29: report The Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting Senate GOP sees early Supreme Court vote as political booster shot MORE (S.C.).

An NBC/Marist poll last week showed McCaskill tied with her Republican opponent, Josh Hawley, at 47 percent.

In North Dakota, the $350,000 ad buy from SLF is hitting Heitkamp as someone who “rubber stamps the Washington liberal agenda.”

The ad highlights Heitkamp’s support for the 2013 comprehensive immigration reform bill as well as for Obama’s Iran nuclear deal in 2015.

It also points to her support for a bipartisan immigration bill backed by Sen. Christopher Coons Christopher (Chris) Andrew CoonsMurkowski: Supreme Court nominee should not be taken up before election Battle lines drawn on precedent in Supreme Court fight Sunday shows - Ruth Bader Ginsburg's death dominates MORE (D-Del.) and the late Sen. John McCain John Sidney McCainCindy McCain endorses Biden: He's only candidate 'who stands up for our values' Biden says Cindy McCain will endorse him Biden's six best bets in 2016 Trump states MORE (R-Ariz.) that provided a pathway to citizenship for more than 1 million immigrants who came to the country illegally as children.

Jordain Carney contributed.