Senate Majority Leader Mitch McConnell is warning his party that President Trump's low approval ratings could erase the GOP's structural political advantages heading into 2018.

The Republicans are defending fewer Senate seats in the midterm, in generally friendlier territory, than the Democrats. Nearly a dozen Democrats are running for re-election in states President Trump won in November.

McConnell said in an interview with the Washington Examiner that this crucial GOP edge wouldn't amount to much if voters are unhappy with Trump next year and decide to elect more Democrats to check his power.

"Don't fall in love with the map," McConnell, R-Ky., said Friday. "The map doesn't win elections."

The party that holds the White House typically loses seats in Congress in midterm elections. But Republicans are confident they might avoid that trend, and add to their 52-seat Senate majority, because most of the expected competitive races are in red states.

Ten Democrats are up for re-election in Trump states, including: Florida, Indiana, Michigan, Missouri, Montana, North Dakota, Ohio, Pennsylvania, West Virginia and Wisconsin.

Just two Senate Republicans appear vulnerable at this point. Only one of them, Sen. Dean Heller of Nevada, is running in a state won by Hillary Clinton.

That's of minor comfort to McConnell, a political tactician with a keen sense of history. The Kentuckian, in office since 1985, wants Republicans to take note.

"If you look at what happened to [former President] Bill Clinton two years in, what happened to [former President] Barack Obama two years in, I'd like to see the president in better shape politically," McConnell said.

Obama's first midterm, in 2010, is an example of what can go wrong for a Senate majority with a favorable Senate map. That cycle marked McConnell's second as the leader of the Republican minority.

In the spring of 2009, with Obama's approval a sky-high 65 percent, the majority 59-seat Democratic majority appeared as though it would pick off a handful of Republicans running for re-election in blue states.

Instead, Republicans flipped six seats, including Obama's old Senate seat in liberal Illinois, plus a seventh, in a special election to replace Sen. Edward Kennedy, D-Mass., who died in office.

That's why Trump's low rating — 41 percent according to the latest RealClearPolitics average — this early in his presidency concerns McConnell, even though Republican voters are still satisfied with his job performance.

Closer than expected campaigns in special House elections for two conservative-drawn seats, one in Georgia, the other in Kansas, have revealed supercharged Democratic enthusiasm, and possibly GOP complacency. Trump improving his political standing could help Republican mitigate these atmospheric challenges next year if they are a factor.

"It would help us take full advantage of what is a good map," McConnell said. "The map alone will not secure the majority."

Republicans currently hold a 24-seat advantage in the House. The party is defending 23 seats that Clinton won last November.