Unlike this year’s U.S. Senate races, mainly being fought in Trump-friendly states, many of the most contested House electoral battles are for Republican-held seats in more Democratic-leaning places. That enhances the odds of Democrats winning control of the House and presents a dilemma for Republican incumbents: Do they strike some distance from President Trump or continue to march in lockstep with him in Congress? Don’t look for any mass Republican defections, even if the probe of any Trump ties to Russia escalates.

Off-year elections usually are about voter intensity. In dozens of contests over the past year, since Trump was elected, the energy has been with Democrats. Most recently, a Democrat won by seven points in a pro-Trump, pro-Republican state legislative district in Sarasota, Florida, in a special election.

The hope for the dozens of embattled congressional Republicans is that by November their base will get energized. Those voters remain strongly supportive of Trump, while the general electorate, despite some recent improvement in the president’s standing, is far less so.

If Republican candidates in blue districts and states run away from Trump, some of the GOP base will stay home; if they wrap themselves around him, they risk energizing Democrats and alienating independents. It’s not an easy choice.

“Buckle up your seat belts and pull up your big-boy pants,” leading Republican pollster Neil Newhouse advises. “This is going to be a tough election.”

A look at Republican-held seats that are the chief Democratic targets — over half of them in states or districts that voted for Hillary Clinton — illustrates the divide on how to walk this tightrope; even most of those taking issue with Trump are doing so very selectively.

Predictably, a dozen Republican incumbents from New Jersey, New York and California voted against the tax-cut bill that socked it to those states by eliminating the federal tax deduction for state and local taxes. On immigration, there are dozens of House Republicans opposing Trump’s hard line, joining Texas’ Will Hurd and California’s Jeff Denham, two of the most endangered congressional incumbents.

Colorado Republican Mike Coffman, facing his toughest challenger in a district carried by Clinton, recently tweeted, “No Shutdowns, No Parades,” warning the president to abandon his plans for an expensive military parade and not to shut down the government. Fighting Trump on a possible government shutdown was a gift for Republican Rep. Barbara Comstock, who represents a Northern Virginia district. She relished a testy exchange at the White House over the issue. She warned him; he fired back.

If Comstock weren’t such a shrewd politician, she’d be a dead woman walking; Trump lost this district by 10 points and is more unpopular today. If there is a blue wave, some Republicans in districts Trump carried easily might be ambushed. These include New York’s Claudia Tenney, who said the Democrats who didn’t applaud during Trump’s State of the Union speech were “un-American and they don’t love our country,” or Ohio’s Steve Chabot who, in defending Trump, sought to smear Special Counsel Robert Mueller and his blue-ribbon staff.

In both those races, Democrats are fielding stronger than expected challengers who may already be crafting commercials and messages that recycle those accusations.

Veteran journalist Al Hunt is a Bloomberg columist.