The number of House incumbents favored to lose reelection is growing as Election Day nears, according to updated forecasts from political analysts.

Inside Elections, one of the three leading groups that analyze and rate congressional races, on Thursday shifted its ratings in 15 House races, all but four of which moved away from the party currently holding the seat.

The ratings shifts showed incumbents on both sides of the aisle in trouble, but Republicans more so. Inside Elections predicts the GOP’s best-case scenario is to hold onto its majority with a one-seat net gain, but Democrats are more likely to pick up seats, as many as nine. Democrats only need a net gain of four seats to flip the House.

House Republicans are not conceding anything at this point.

“I’m very confident that Republicans will hold the House majority,” National Republican Congressional Committee Chair Richard Hudson said Thursday on CNN. “We still have a pathway to grow the majority.” 

Inside Elections moved four Republican incumbents’ races from the toss-up column to tilt Democratic: Reps. Anthony D’Esposito and Marc Molinaro in New York, Mariannette Miller-Meeks in Iowa and Lori Chavez-DeRemer of Oregon.

Sabato’s Crystal Ball, another congressional race forecaster, was the first to rate Mr. D’Esposito’s race as favoring Democrats, along with fellow New York Republican Rep. Brandon Williams and Nebraska Republican Rep. Don Bacon.

Inside Elections has also considered Mr. Williams and Mr. Bacon underdogs for some time.

The Cook Political Report continues to rate all the aforementioned Republican incumbent races as toss-ups — except for Mr. Williams’ race, which Cook also believes favors Democrats. 

But Cook did include Mr. D’Esposito and Ms. Chavez-DeRemer, along with Mr. Williams and Mr. Bacon, on a list of the top 10 most vulnerable incumbents it released Wednesday. 

Cook’s most vulnerable list had six Republican incumbents, showing the current majority party facing stronger headwinds than Democrats, who only had four incumbents on the list. 

The other Republicans on the list were California Reps. John Duarte and Mike Garcia.

The four Democrats on the most vulnerable list were Reps. Mary Peltola of Alaska, Yadira Caraveo of Colorado, Jared Golden of Maine and Marie Gluesenkamp Perez of Washington. 

Ms. Peltola “may be the most endangered Democratic incumbent on the map,” Inside Elections said, as it moved her race from a toss-up rating to tilt Republican.

The presidential race is expected to have down-ballot impacts in many of these races, as most of the Republicans in danger are running in districts President Biden carried in 2020, while the most vulnerable Democrats are from districts former President Donald Trump won that year.

It wasn’t just more incumbents’ seats that were favored to flip.

Inside Elections moved the open seat in Michigan’s 7th District, which Rep. Elissa Slotkin is vacating to run for Senate, from a toss-up to tilt Republican.

The Republican candidate is Tom Barrett, an Army veteran who lost to Ms. Slotkin by 5 points in 2022. He faces Democrat Curtis Hertel, who served in the state Senate and as Gov. Gretchen Whitmer’s director of legislative affairs.

Inside Elections’ other ratings shifts did not move anyone into underdog territory.

But the forecaster did move Republican Reps. Juan Ciscomani of Arizona and David Valadao of California from slightly favored to win to toss-up status, and Wisconsin Rep. Van Orden moved a level down to tilt Republican. 

Democratic Reps. Mike Levin of California and David Trone of Maryland were moved from “likely” to win to “lean” races in which they’re only slightly favored.

It wasn’t all bad news for incumbents. Inside Elections moved two incumbent races into safer territory: Iowa Republican Rep. Zach Nunn’s race shifted from a toss-up to tilt Republican, and New Mexico Democratic Rep. Gabe Vasquez moved from tilt Democratic to lean Democratic.

Open seats Democrats currently hold in Virginia’s 7th District and Michigan’s 8th District also shifted from toss-ups to tilt Democratic. 

That’s bad news for perennial Republican candidate Paul Junge, who has lost the last two races in Michigan’s 8th District.