Buoyed by a string of Republican retirements and President Donald Trump's persistently low approval rating, Democrats are increasingly hopeful about their chances for a midterm election wave that would give them control of the House and deliver a blow to the president.

The number of Republicans bowing out rather than bearing down for tough races is the latest worrisome sign for the GOP. Combine that with Trump's ability to unite Democrats in opposition and historical headwinds, and some Democrats are optimistic.

"We don't have an Obama figure energizing us; we have Trump energizing us," said Democratic Rep. Gerald Connolly of Virginia, as he described standing-room-only gatherings at local Democratic events. "Who is the D? Show me who the D is, so I can vote for them," he said of voter sentiments. "I think it's shaping up into that kind of election."

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Rep. Ben Ray Lujan of New Mexico, who is leading Democrats' House campaign effort, said there is a "clear path to a majority," something he said he never saw in 2016.

Indeed, Trump's job approval rating — a key indicator in midterm elections — lags below 40 percent in most polls, and marks for Congress are half that. Since Trump's inauguration, Democrats have won state legislative elections across the country, reclaimed the Virginia governor's seat by a surprising 9-percentage-point margin and managed an upset Senate victory in GOP-dominated Alabama, albeit with the help of a Republican nominee accused of sexual misconduct with teenage girls.

The next test is in Pennsylvania, where a March special election to replace Republican Rep. Tim Murphy, who resigned amid allegations he asked a woman he was having an affair with to get an abortion, will become another test of momentum. Trump is expected to campaign for the Republican candidate, state Rep. Rick Saccone.

Even House Speaker Paul Ryan concedes that his party may have to "buck history" for him to keep his job, though he maintains that voters will reward Republicans after their sweeping tax cuts. "The reason I feel confident and comfortable is because we ran on a set of ideas and policies, we're now executing those ideas and policies, and the results are proving themselves," Ryan said Friday in Wisconsin.

Nonetheless, the environment has contributed to a steady stream of Republican retirements. This week, Ed Royce and Darrell Issa of California, two of the more vulnerable GOP members, announced they would not run again. Altogether, 31 House Republicans have announced their retirements so far, ahead of a typical election-year pace and giving rise to comparisons with 1994, 2006 and 2010, the last three times that voters flipped control of the chamber.

Ohio Rep. Steve Stivers, who heads the GOP's House campaign operation, says he believes the retirement run is near its end, but he and other Republicans concede that the later the retirements, the harder it is for candidates to step in and build the campaigns necessary to win.

National Democrats, meanwhile, are targeting 91 House districts — a list that covers nearly all the GOP vacancies — and they say they have "viable" candidates in 87 of those districts.

Since World War II, the president's party has never gained seats when the president's job approval rating is below 50 percent, a threshold Trump has yet to reach. Gallup polling of the presidents' approval rating in the week before midterm elections offers some guidance.

Barack Obama watched Democrats lose 63 seats in 2010 with a 45 percent rating; Bill Clinton lost 53 seats at 46 percent in 1994; and Ronald Reagan lost 28 seats at 42 percent in 1982. Jimmy Carter managed the narrowest losses in 1978, losing 11 seats with a 49 percent approval rating.

"The real question is how many seats the president's party loses," Ayres said, and whether Ryan can "limit the damage."

Republicans find comfort in district lines that GOP-run legislatures drew after 2010. The boundaries dilute Democratic voting strength by concentrating the party's most reliable voters in fewer districts. Those advantages explain part of the GOP's current 24-seat margin in the 435-representative chamber.

"Our districts got drawn in a way where, yeah, there's 20 or 30 competitive seats out there, always will be, but most of them are pretty hard to flip," said Rep. Greg Walden, R-Ore., who oversaw GOP efforts in House races in the previous two election cycles.

But even partisan boundaries might not withstand a genuine wave election. Democrats are leading by double digits in many "generic ballot" polls, which ask voters whether they prefer a Democratic or a Republican congressional candidate. Pollsters say such a national generic ballot lead is likely enough to overcome GOP advantages from gerrymandering.

Democratic momentum showed up throughout 2017, even in four House special elections to replace Trump executive branch appointees in GOP strongholds. Republicans swept the four, but Democrats managed double-digit swings from the November 2016 results in each instance.

"For years, you've seen independents in red states and swing states acting more like Republicans," said Democratic pollster Zac McCrary. "That trend is reversing."

Democrats' richest targets are the 23 GOP-held districts where Democratic presidential nominee Hillary Clinton outpaced Trump. Most of those are in suburban areas, including several in states where Democrats believe they can capitalize on new tax provisions that cap filers' deductions for state and local property taxes at $10,000.

Issa and Royce represent such districts, as do many incumbent Republicans in California, Illinois, New Jersey, New York and Pennsylvania.

Several of those districts are among the ones that Ryan's outside political operation is working to defend. The Congressional Leadership Fund, an independent political action committee, has opened field offices in 27 GOP-held districts so far. The group has set a fundraising goal of at least $100 million for the cycle, more than either the Democrats' or Republicans' official House campaign committees raised during the first 11 months of 2017.

Rep. Leonard Lance, R-N.J., represents one such district where the group has set up shop. Lance has seven Democrats vying to challenge him. Clinton carried the district by about 3,800 votes. Lance won his fifth term in 2016 by 38,000. "I like that extra zero," he said.

Democrats are "running against President Trump," Lance said while emphasizing his votes against the GOP's final effort to repeal "Obamacare" — though he's voted for repeal before — and his opposition to the new tax law. "I vote my conscience here," Lance said, "and I will be judged based upon my vote."