Senate Majority Leader Mitch McConnell on Tuesday called the 2018 Senate map “very challenging,” pointing out that the party of the White House almost always loses seats in the president’s first midterm elections and ticking off nine races he described as too close to call.

Mr. McConnell told reporters in Louisville he’s hopeful the GOP can hold onto what is currently a slim 51-49 majority in the Senate.

“You know, we’ve got a storm headed toward the coast … we know the wind’s going to be in our face. We don’t know whether it’s going to be Category 3, 4, or 5,” the Kentucky Republican said.

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

Mr. McConnell listed races in Arizona, Nevada, Tennessee, Montana, North Dakota, Missouri, Indiana, West Virginia and Florida as “dead even” or “too close to call.”

“Every one of them like a knife fight in an alley — just a brawl in every one of those places,” he said. “I hope when the smoke clears that we’ll still have a majority in the Senate.”

Republicans are defending GOP-held seats in Arizona, Nevada, and Tennessee, while Democrats are playing defense in the other six states Mr. McConnell named.

He also said by all indications Sen. Ted Cruz, Texas Republican, is in a competitive race against Democratic Rep. Beto O’Rourke.

Mr. McConnell said the chamber’s role in confirming executive branch appointments would be adversely affected if Democrats retook control.

“The Senate’s in the personnel business — the House is not,” he said.

Political handicappers have given Democrats much better odds at retaking the House, where they need to pick up roughly two dozen seats to regain control, compared to the Senate, where they have fewer realistic pick-up opportunities compared to Republicans.

Mr. McConnell’s comments came on the same day that the Senate Leadership Fund, a super PAC backing Senate Republican candidates, announced new advertising campaigns targeting Democratic Sens. Joe Donnelly of Indiana, Claire McCaskill of Missouri and Heidi Heitkamp of North Dakota.

The group also announced ad campaigns targeting Democratic Senate hopefuls Phil Bredesen in Tennessee and Jacky Rosen in Nevada. The group said the buys across the five states total close to $6 billion.

The group also announced an $800,000 buy targeting Democratic Sen. Joe Manchin of West Virginia last week.