More conservatives are likely to win seats in the House in this fall’s midterm elections, which could pose a challenge for Republican leaders. (MLADEN ANTONOV/AFP/Getty Images)

A new band of combative conservatives is likely to win House seats next week, posing a fresh challenge for Speaker John A. Boehner (R-Ohio) and his leadership team as they seek to govern an expanded GOP majority next year.

Six to eight new lawmakers are likely to replace incumbent Republicans in deep-red districts, primarily in the South. Most of them, such as Gary Palmer of Alabama and John Ratcliffe of Texas, are backed by the tea party movement and will be more likely than their predecessors to oppose GOP leaders on key legislation.

Other Republican wins are expected in blue states such as California, Florida, Illinois and New York, where voters take a dimmer view of the tea-party-style politics that has dominated the House in recent years. New lawmakers from those areas are more likely to temper their partisanship ahead of the 2016 elections, when congressional Democrats are expected to fare better.

Overall, party leaders and election forecasters generally agree that the GOP will add eight to 10 seats to its 234-seat majority in the House. The party also stands a good chance of seizing control of the Democratic-run Senate.

“Republicans are going to gain seats — we just don’t know how many yet, even this close to Election Day,” said Nathan Gonzales, deputy editor of the nonpartisan Rothenberg Political Report, which closely tracks congressional races.

Election Lab: See our current forecast for every congressional race in 2014

Part of the uncertainty in the House stems from the enormous cash advantage amassed by Democrats in recent months that has allowed endangered incumbents to keep pace with an influx of attack ads paid for by Republicans or GOP-leaning super PACs.

Even so, Gonzales said, “Democrats might have a problem this cycle that money can’t fix.”

GOP leaders launched a “Drive to 245” fundraising campaign last spring in hopes of winning the largest House majority in modern history. Some Republicans privately predicted that they could snatch 25 new seats.

Democrats, meanwhile, would need at least 17 seats to retake control. House Minority Leader Nancy Pelosi (D-Calif.) said in a Washington Post interview this summer that she wanted to pick up 25 seats.

Both sides now admit that they will probably come up short.

The Post’s Election Lab predicts with 99 percent certainty that Republicans will maintain control of the House and gain eight seats. The forecasts from Rothenberg and the nonpartisan Cook Political Report expect that no more than 10 seats will change hands from Democrats to Republicans.

Officials at the National Republican Congressional Committee privately predict a gain of four to nine seats, within striking distance of their original 11-seat goal.

Rep. Greg Walden (Ore.), who chairs the NRCC, said in a statement that Democrats “are finding themselves increasingly on defense” in districts that President Obama previously won. “Americans are turning to Republican candidates in these races because they know we’re the only ones who will provide a check and a balance on the president’s misguided policies,” he said.

Democrats began the year believing they could chip away at the GOP’s House majority because of Congress’s widespread unpopularity. They touted Obama’s plans to hold fundraisers and campaign for candidates where needed.

But Obama’s sustained unpopularity and the traditional midterm drop-off among Democrats have spoiled any hopes for such gains. After building a cash advantage over the summer to target vulnerable Republicans, House Democrats have retreated in recent days to protect embattled colleagues and other once-safe seats in California, Hawaii, Iowa, West Virginia and Nevada, where the NRCC and GOP-aligned super PACs are airing new attack ads.

In a conference call Tuesday, Democratic leaders begged colleagues to fork over more cash to help nine candidates on a “Red Alert List” — six at-risk incumbents and three challengers they hope can pick up GOP seats. During the call, members pledged to donate about $500,000 by Friday.

“The good news for us is that by this point in 2010 many, if not most, of our competitive incumbents were already down and out,” Rep. Steve Israel (N.Y.), who chairs the Democratic Congressional Campaign Committee, said in an interview Tuesday. “This year, we prepared for the climate we have now — not a single one of our incumbents is a lost cause.”

Democratic targets include embattled Rep. Michael G. Grimm (R-N.Y.), who is the subject of a federal corruption probe; Rep. Steve Southerland II (R-Fla.), a tea-party-backed lawmaker struggling to stay ahead; and Rep. Gary G. Miller (R-Calif.), a perennial target. They also hope to win open seats in Arkansas, Iowa, Maine and West Virginia.

Republicans are focused on winning in 16 districts that Obama narrowly carried in 2012 — a list that includes Democratic Reps. Ami Bera and Scott Peters of California, Timothy H. Bishop and Daniel Maffei of New York, and Brad Schneider and William Enyart of Illinois. They’re also targeting six seats held by Democrats in districts that Republicans won in recent presidential contests. In those districts, Reps. John Barrow (Ga.) and Nick J. Rahall II (W.Va.), are most vulnerable and are facing a fresh barrage of attack ads this week that use video of Obama recently saying that “every single one” of his policies is on the ballot.

Two House Republicans from Georgia who did not win the GOP nomination for an open Senate seat will be replaced by more conservative figures: Barry Loudermilk is expected to succeed Rep. Phil Gingrey and has said that he will not support Boehner. Jody Hice is poised to replace Rep. Paul C. Broun and has said that he is seeking “new leadership with a backbone.”

Glenn Grothman, likely to succeed retiring Rep. Tom Petri (R-Wis.), calls himself “more conservative than John Boehner.” Then there’s David Brat, who stunned the nation by defeating former House majority leader Eric Cantor (R-Va.) over the summer.

Overall, though, most of the House will remain the same — in large part because of elaborately drawn congressional maps designed to silo voters into separate camps.

“I’m looking at a variety of races that are simply censuses that count how many Democrats or Republicans live in a different area,” said Dave Wasserman, who tracks House races for the Cook Political Report. “We’ve really moved to a more parliamentary system of congressional elections that has left the parties with very few targets.”