Republican senators in purple states are watching the GOP presidential primary with more than a passing interest. Their jobs—and the Republicans’ 54-46 Senate majority—depend on it.
A Senate candidate can run only so far ahead of the party’s presidential ticket. Take the 2000 election, when Republicans’ 54-46 majority was eroded by a net loss of four seats. Three incumbent GOP senators (Delaware’s Bill Roth, Michigan’s Spencer Abraham and Washington state’s Slade Gorton) lost in states carried by Al Gore, despite running between 1.8% and 4% ahead of George W. Bush. They outpolled their party’s nominee, but not by enough to win.
Four years later, President Bush’s coattails helped his party rebuild its majority. Ten Republican senators were elected while trailing the president’s performance in their states, for a net GOP gain of four seats.
In the past two presidential contests, the Republican ticket’s downward pull on the party’s Senate candidates was pronounced. In 2008 Republican incumbents lost in New Hampshire and Oregon, despite running ahead of John McCain. The GOP incumbent also lost in Minnesota, where Democrat Al Franken ran more than a dozen percentage points behind Barack Obama.
In 2012 Republicans lost Senate races in Indiana, Missouri, Montana and North Dakota, all of which Mitt Romney carried, as well as in Ohio and Virginia, states where Mr. Romney trailed only narrowly.
How does that translate to 2016? The performance of the presidential candidate in battleground states could make or break incumbents’ re-election bids. Five GOP senators who will be on next year’s ballot— Kelly Ayotte of New Hampshire, Pat Toomey of Pennsylvania, Rob Portman of Ohio, Ron Johnson of Wisconsin, and Mark Kirk of Illinois—represent states twice carried by President Obama. Understanding the challenge this presents, each has organized early, raised substantial funds and worked to court independent and swing voters.
Still, they must either outpoll the GOP presidential candidate by enough to win or hope that the ticket can carry their state. The same goes for the eventual Republican candidate for the Senate seat in Florida being vacated by Marco Rubio.
A weak presidential nominee could harm the re-election chances of GOP Sens. Richard Burr of North Carolina, Roy Blunt of Missouri and John McCain of Arizona. Those states were carried by Mr. Romney in 2012, but they also include a healthy number of ticket-splitters.
The nominee’s performance also matters to candidates trying to pick up seats held by Democrats. In Nevada, the GOP has recruited a top-flight Senate candidate in Rep. Joe Heck. Though Mr. Obama won the state twice, voter registration numbers and local election results show a trend toward the GOP that could flip retiring Sen. Harry Reid’s seat.
Then there is Colorado. Democratic Sen. Michael Bennet has dreadful poll numbers. The state’s voters give Mr. Obama a worse approval rating than he receives nationally, and they appear cool to Hillary Clinton. One reason may be that the former secretary of state advised the president to close the Guantanamo Bay detention facility and transfer the remaining Islamic terrorists to a federal prison in Colorado.
If Republicans win the White House but lose the Senate, Democrats could give the new president a devil of a time, especially in filling Supreme Court vacancies. A bad presidential nominee could even jeopardize the GOP’s comfortable House majority: 26 Republicans hold districts carried by Mr. Obama in 2012.
There are also 16 seats in which Mr. Romney won by less than 3% of the vote, and nine more that he took by less than 5%. Purple states and districts differ from deep-red Republican ones, with many more independents and swing voters, and a greater share of nonwhites and millennials.
Outside of the White House, Republicans are dominant, with majorities in both branches of Congress, governorships and state legislatures. To maintain or build on that achievement requires nominating a presidential candidate who can appeal to purple-state voters. There are candidates running who can do that. But there are also several who, if they win the nomination, could destroy these Republican gains.
Mr. Rove helped organize the political-action committee American Crossroads and is the author of “The Triumph of William McKinley: Why the 1896 Election Still Matters,” out Nov. 24 from Simon & Schuster.