Voters headed to the polls Tuesday to decide whether Democrats or Republicans will control the House and Senate in January, with the outcome of key, toss-up and swing district races a major factor in the outcome.  

To win a House majority, Democrats would need to hold their own vulnerable seats while winning a net total of three to four seats to win an outright majority of 218 seats in the 435-seat chamber.

Republicans currently hold a 220-215 majority with House Speaker Mike Johnson, R-La., holding the gavel and running the chamber.

House Democrats are expected to elevate House Minority Leader Hakeem Jeffries, D-N.Y., to speaker if they win control.

A narrow majority would leave either party with little room for defections, which makes leadership decisions, spending negotiations and major legislative priorities difficult to navigate. 

The Cook Political Report's October 30 ratings identify 18 toss-up races, including 14 currently held by Republicans and four held by Democrats. 

Pennsylvania's 1st district is a tight race to track as a potential seat that could flip to the Democrats. A New York Times/Siena poll conducted October 16 through 22 has Democrat Bob Harvie with a slight lead over Republican incumbent Brian Fitzpatrick, 48% to 46%.

Pennsylvania's 10th district is a competitive race. Republican incumbent Scott Perry is tied with Democrat Janelle Stelson at 48%, according to a New York Times/Siena poll conducted October 16 through 22.

Democrat Shannon Taylor is locked in a tight race with Republican incumbent Rob Wittman in Virginia's 1st district: A Public Policy Polling survey conducted October 21 through 22 showed the race essentially tied at 47% to 46%. 

Some other competitive races that remain toss-ups include Iowa's 1st and 3rd districts, where Republican incumbents Mariannette Miller-Meeks and Zach Nunn are defending seats. New York's 17th district is another race to watch closely. Republican Mike Lawler is running against Democrat Mondaire Jones in a race that remains a toss-up.

There are 13 Republican-held districts that Cook Political Report identifies as leaning Democratic as of Sunday, including Virginia's 2nd district. Democrat Elaine Luria is leading Republican incumbent Jen Kiggans, 46% to 43%, according to a Christopher Newport University poll conducted October 1 through 10.

Other races in that category include the open seat currently occupied by Darrell Issa, R-Calif. San Diego City Council member Marni von Wilpert is running against Republican Jim Desmond in the general election. Incumbent Mariannette Miller-Meeks, R-Iowa, is running against Christina Bohannan. Her race has been moved to lean Democrat as well.

The overall polling picture gives Democrats a national advantage heading into Election Day.

A YouGov survey conducted October 23 through 26 shows Democrats at 50% and Republicans at 41%. An Ipsos survey conducted October 22 through 26 gives Democrats a wider advantage at 53% compared to Republicans at 44%. 

An NPR/PBS poll conducted October 22 through 25 has Democrats at 52% compared to Republicans at 44%.

Election results on Tuesday night may not immediately settle which party will control the chamber, given that close races, particularly in states with lengthy ballot-counting procedures, could take days to fully resolve.