Congressional Leadership Fund prepares to battle Democrats in midterm election

House Republicans’ main super political action committee said Thursday it’s expanding its efforts to include several districts that the GOP easily carried in 2016, signaling the party is increasingly concerned about the prospect of an anti-Trump wave gathering for November’s midterm election.

The Congressional Leadership Fund also said it will take steps to defend key Republican House leaders and committee chairmen, in what appears to be an attempt to build a firewall to limit losses and prevent a Democratic takeover.

“CLF is taking nothing for granted as we focus on our mission to maintain the House Republican majority,” said executive director Corry Bliss.

His group said it will open field offices in districts held by Rep. Cathy McMorris Rodgers of Washington, the No. 4-ranking House Republican, as well as Rep. Peter Roskam of Illinois, who chairs a tax panel on the powerful Ways and Means Committee.

And while the group is ramping up efforts to defend perennially embattled Republicans such as Rep. Mike Coffman of Colorado and Rep. Will Hurd of Texas, it is also looking at Rep. Ed Royce of California, the House Foreign Affairs Committee chairman who won his 2016 race by about 14 percentage points, and Rep. Andy Barr of Kentucky, who won by more than 20 points in 2016.

Getting involved in those races is an acknowledgment that Republicans are worried about the extent of losses, said GOP strategist Ford O’Connell.

“They’re very realistic about what’s going on here,” Mr. O’Connell said. “They’re telling you straight out, we know that Democrats are fired up, we know it’s going to be a hard slog — we’re going to be realistic about it.”

Democrats need to pick up a net of two dozen seats this year to retake control of the House.

The CLF says its national field program now boasts offices in 27 districts.

The American Action Network, a separate pro-House GOP political group also helmed by Mr. Bliss and friendly to Speaker Paul D. Ryan, is also weighing in on some of the same races.

That group recently launched a $2 million TV and digital ad campaign in 23 congressional districts to thank House Republicans who voted for the GOP’s $1.5 trillion tax-cut package last month.

There are currently 23 Republican members who represent districts that broke for Hillary Clinton in the 2016 election, and more than a dozen of them have been explicit targets of one or both of the Bliss-aligned political groups.

Democrats hope to go beyond those Clinton districts and win Trump districts as well, pointing to the president’s cratering support.

Some recent public polling has given Democrats a double-digit edge over Republicans when voters are asked which party they plan to vote for in the House elections this year. Analysts said that’s about the level of polling needed for Democrats to have a realistic shot at a House takeover.

Shortly after CLF’s announcement Thursday, the main House Democratic super PAC announced it raised close to $15 million in 2017 — up from $8 million in 2015, the previous off-year — and started the year with more than $11.4 million on hand.

“Thanks to a record effort last year, we will be able to do more in 2018 than ever before to help elect Democrats that are working to lift up middle-class families and combat [House Speaker] Paul Ryan’s toxic agenda,” said Charlie Kelly, executive director of the House Majority PAC.

Super PACs, which unlike traditional candidate and party committees can raise and spend unlimited amounts of money, have taken on an increasingly prominent role in recent election cycles.

Mr. O’Connell put the odds of a Democratic takeover of the House right now at slightly better than 50-50, but predicted the messaging would be less about Mr. Ryan than the president.

“They’re going to run on impeach Trump — everything about Trump,” he said.