The battle to control Congress is heating up, as Sunday marks the final 100 day stretch to Nov. 3, with Democrats leading in the polls but the outcome still in doubt after Republicans gained more from redistricting battles. 

Republicans hold slim majorities in both the upper and lower chambers of Congress, while polls suggest a possible victory by Democrats in November. AJuly 23 Emerson poll revealed an 11-percentage point lead by Democrats on the generic ballot, 53% compared with 42% favoring Republicans, as President Donald Trump's approval rating lands at 39 percent. 

However, other polls show a much closer race. In a Washington Post-Ipsos poll, also released Thursday, voters said they were more likely to vote Democrat if the U.S. House of Representatives race were held today. This was only by a three-percentage point lead of 48% Democrat vs. 45% for Republicans. This is a slightly closer margin than when the same poll was conducted in April, when 49% of voters preferred Democrats and 44% selected Republicans.  

The president's endorsement power in Republican primaries remains strong, with a recent sweep in Arizona on Tuesday. Trump's endorsed congressional nominees so far have won every primary, according to data from Ballotpedia reviewed by USA TODAY, and his power will continue to be tested in coming weeks as more states hold primaries. 

Trump's success in ousting incumbents was on full display when Trump-endorsed Republican opponents beat Sens. John Cornyn (Texas) and Bill Cassidy (Louisiana) and Rep. Thomas Massie (Kentucky) for the nominations to their current seats. 

The economic strain posed by the war in Iran remains a liability for Republicans, who hold power in the White House, Senate and House of Representatives. On July 21, Speaker of the House Mike Johnson, R-Louisiana, broke with the president, calling for an end to the military action.  

"An unpopular war with Iran will put more political pressure on Republican congressional candidates the longer it persists," James Lindsay, a senior fellow in U.S. foreign policy at the Council on Foreign Relations and former director on the National Security Council, wrote in April. 

Redistricting will benefit Republicans

Despite Democratic leads in the polls, the rural bias in the Senate and recent redistricting wins across the country give Republicans a built-in advantage. Republicans currently control the House at 218 seats, compared to the Democrats’ 212 seats, one independent seat and four vacancies. Political analyst Nate Cohn for the New York Times estimated that Democrats would need to win the national popular vote by roughly 3.1 points to just barely flip the House of Representatives. 

Eight states − Alabama, Florida, Louisiana, Missouri, North Carolina, Ohio, Tennessee and Texas – installed new congressional maps that could benefit Republicans ahead of the midterms. Potentially, Republicans could pick up an additional 10 seats in the House of Representatives, according to Ballotpedia in June. A different prediction from the Center for Politics estimated that Republicans added a five- to nine-seat advantage.

"This is clearly an improvement for Republicans, but the House map is not red enough to protect Republicans from what appears to currently be a bad political environment," Kyle Kondik, a national political analyst, wrote in June in the University of Virginia's Center for Politics.

Only two states, Utah and California, successfully approved new maps that could give Democrats extra seats. In June, the Cook Political Report rated 18 races – 14 of which involve seats currently held by Republicans – as toss-ups. 

These 10 gerrymandered states account for 42% of the representatives in Congress' lower chamber, according to the Center for Politics.

Senate seats to watch 

Both parties are hoping to pick up additional seats in the Senate, where the Republican majority sits at 53, over the 45 Democrats and two independents who tend to caucus with the minority. Here are some hot races that both parties are pouring resources into: 

• Maine: Incumbent Republican Sen. Susan Collins, who has served since 1997, will take on Democrat Troy Jackson. On Saturday, Maine Democrats voted to nominate Jackson for the spot previously filled by Graham Platner, who ended his campaign after sexual assault allegations surfaced against him, which he denies. 
• North Carolina: Democrats hope to flip the seat currently held by Republican Sen. Thom Tillis. Former Democratic Gov. Roy Cooper is leading by 14 points against Michael Whatley, former chairperson of the Republican National Committee, according to a June Catawba College-YouGov survey. 
• Michigan: On the Republican side, former U.S. Rep. Mike Roger is the only candidate. On Aug. 4, Michigan Democrats will vote to nominate Abdul El-Sayed, a Detroit-based public health officer, or U.S. Rep. Haley Stevens, a former Obama administration official. Recent polling puts Stevens in the lead. 
• Alaska: The Trump-endorsed Republican incumbent Sen. Dan Sullivan is challenged by Democratic former Rep. Mary Peltola, who led in a spring poll by the Alaska Survey Research. A nonpartisan primary vote on Aug. 18 will advance the top four candidates to November’s general election with a ranked-choice voting system. Also on the primary ballot: a second man by the name of Dan Sullivan, and 14 other Alaskans. 
• Georgia: Incumbent Democratic Sen. Jon Ossoff leads by nine percentage points to Republican challenger Rep. Mike Collins, according to a July 23 AARP poll. Trump is advocating for Collins, as Ossoff is a known fierce critic of the president. Just on Wednesday, July 22, Trump compared Ossoff to "Pinky Herman" and called the senator a "Democrat community," at a Georgia rally where the president campaigned for Collins.