Last week's retirement announcements from two long-serving House Republicans have made Democrats even more optimistic that they can engineer a string of victories in the midterm elections that could give them the majority again.

Reps. Ed Royce and Darrell Issa, two California Republicans, added their names to a growing list of retirements, although there was talk that Issa may run again in a nearby, more Republican-leaning district.

Still, 31 incumbent GOP lawmakers have decided not to run again, and about one-third of them hail from districts that are at least competitive for a takeover by the Democrats. They need to win 25 new seats to overtake the GOP in November.

Issa’s San Diego district and Royce’s Los Angeles and Orange Country district are particularly appealing to Democrats. Voters in both districts chose Hillary Clinton over Donald Trump in 2016, and Issa was barely re-elected.

Analysts now rate both districts slightly in favor of a Democratic takeover in November, and some Democrats believe the majority is again in sight after they lost it in 2010.

“A bigly blue wave is coming this November,” Rep. Ted Lieu, vice chairman of the House fundraising arm, tweeted after Issa announced his retirement on Wednesday.

Midterm elections are almost always tough on the party of the president, and this year could prove to be particularly brutal. Poll numbers show Trump is having difficulty winning the approval of voters, despite signing a new law reducing almost everybody’s tax bill, eliminating the unpopular Obamacare mandate, and accomplishing other goals aimed at boosting the economy and jobs.

Gallup’s weekly job approval rating showed 58 percent of voters disapproved of the job Trump is doing, while just 38 percent approved of the president’s performance.

House Minority Leader Nancy Pelosi, who led the party to a majority more than a decade ago, is watching the numbers closely.

“Oh my gosh, he’s under 40,” Pelosi said on Thursday, referring to Trump’s low approval rating.

“History is on our side,” she added. “When President Clinton's numbers went down, they won. When President Bush's numbers [went down], we won. When President Obama's numbers went down, they won.”

Analysts speculate the surge in retirements is a sign lawmakers are fearful that voter disapproval of Trump will drag down the ticket and cause major Republican losses down the ballot in November.

The head of the House GOP campaign arm disagreed. Rep. Steve Stivers, R-Ohio, who heads the National Republican Congressional Committee, said the retirements are not provoked by Trump, but rather the GOP term limit system for committee chairs.

Royce is about to conclude a six-year term as the top Republican on the House Foreign Affairs Committee, while Issa’s term as chairman of the House Oversight and Government Reform Committee ended two years ago.

When the immediate prospects of chairing a new panel dims, many lawmakers retire.

“When their six years is up, they tend to leave, and that opens up some seats,” Stivers said. "We see that time and again."

Stivers said the party already has found “good recruits” to run in both districts and pointed to the GOP’s victory in all five special election House races this year. The congressional district map also favors Republicans, Stivers said.

“We clearly have to defy history,” Stivers said. “But I feel comfortable we are going to be able to do it. We have great candidates, solid lines, and we’ve got the money that it takes to win.”