WASHINGTON — A key question looming for vulnerable Republican senators is whether they will survive this election if Donald Trump loses big.
With 11 weeks until Election Day, Trump’s declining standing in the polls has GOP Senate candidates preparing for the worst, and they’re maneuvering now to put as big a margin as they can between themselves and the top of the ticket.
Some strategists foresee a historic Trump loss and the need to outrun the presidential nominee by at least 5 to 10 percentage points in states like Florida, New Hampshire, Pennsylvania, and Ohio if Senate incumbents are to prevail in November.
Such margins could be achieved, but it would not be easy, and most Republicans say there’s a limit to how big Trump could lose without taking down nearly every vulnerable congressional incumbent.
Already, GOP senators in Illinois and Wisconsin are widely considered unlikely to survive in November. Few expect the solidly Republican House to change hands, but losses of as many as 15 or 20 seats are forecast.
Most GOP strategists now view a Trump loss to Democrat Hillary Clinton as a certainty, and their only question is how big.
‘‘I’m more likely to think that it’s going to be historic than that it’s going to be close,’’ said GOP strategist Rob Jesmer, formerly executive director of the National Republican Senatorial Committee.
Democrats need to pick up four Senate seats to claim the majority if they hang onto the White House, since the vice president breaks tie votes.
While Republicans are playing defense in more than a half-dozen heavily contested states, Democrats have only one seat at risk, in Nevada, where Senate minority leader Harry Reid is retiring.
Instead Democrats are expanding their list of pickup opportunities, adding Indiana by recruiting former senator Evan Bayh and working on North Carolina and Missouri.
Republicans take comfort in the fact that for now, incumbents including Senators Rob Portman of Ohio, Pat Toomey of Pennsylvania, and Marco Rubio of Florida are running well ahead of Trump in their states. Polls show Trump losing all three states, even as Rubio and Portman are ahead of their rivals and Toomey is close to Democrat Katie McGinty.
‘‘Strong Senate candidates can stand on their own even if they are fighting against the tide, and there are numerous examples of where that’s happened,’’ said Steven Law, Senate majority leader Mitch McConnell’s former chief of staff who now works with super PACs dedicated to helping Republicans.
In 2012, Barack Obama’s reelection bid was a drag on Democrats in red states. Nonetheless, Democrat Heidi Heitkamp won in North Dakota, even as Obama lost the state by 20 points, and Democrat Jon Tester won in Montana, where Obama lost by 14 points.
In 1996, GOP presidential nominee Bob Dole was headed for such a clear loss that Republicans began to run ads calling on voters to keep them in control of Congress to provide a check on Bill Clinton in the White House.
It worked, as Republicans won Senate seats that year despite losing the White House. In Arkansas, Clinton won his home state by 17 percentage points but a Republican won the Senate race.
This year, too, Republican senators and strategists are openly discussing split-ticket voting and messages aimed at persuading voters that a Hillary Clinton victory would only increase the imperative for a Republican Congress to act as a check on issues including Supreme Court nominations. Strategists anticipate that a clear pivot to that message could come after Labor Day.
