Strong support from women and independents is fueling Democrats' large early advantage ahead of this year's congressional elections, a sign that two groups that have recoiled from Donald Trump's presidency will play a decisive role in November, according to a new Washington Post-ABC News poll.

The ongoing government shutdown and rising economic optimism are just two factors that could shuffle preferences over the nine months before Election Day, with Republicans hoping to take more credit for economic growth and cast Democrats as anti-Trump obstructionists.

By 51 percent to 39 percent, more registered voters say they would support the Democratic candidate in their congressional district over the Republican. Democrats' 12 percentage-point advantage on this "generic ballot" question is the largest in Post-ABC polling since 2006, although it is slightly larger than other polls this month.

Judging from past elections, Democrats are expected to need a six- to eight-point advantage in national support this fall to gain the 24 seats needed win control of the House. Election handicappers say a Democratic takeover is possible, but not yet likely. Democrats would fall five seats short even if they won all contests the Cook Political Report classifies as solidly Democratic, leaning Democratic or toss-ups.

The midterm elections loom heavily over the federal government shutdown that began Saturday, after the poll was completed. The Post-ABC poll found more Americans saying they think Trump and Republicans were responsible for the shutdown, although Republican leaders have expressed confidence that Democrats will be blamed for insisting on concessions for young undocumented immigrants before backing a funding bill.

The Post-ABC poll finds Democrats holding a 57 percent to 31 percent advantage among female voters, double the size of Democratic presidential nominee Hillary Clinton's margin in the 2016 election. Nonwhite women favor Democrats by a 53-point margin, somewhat smaller than Clinton's 63-point advantage over Trump in 2016. But white women have moved sharply in Democrats' direction, favoring them over Republicans by 12 points after supporting Trump by nine points in 2016 and Republican candidates by 14 points in the 2014 midterm election, according to network exit polls.

Partisan loyalty is strong, with Republican and Democratic candidates garnering support from at least 9 in 10 of their fellow partisans, but self-identified political independents favor Democrats by a 16-point margin, 50 percent to 34 percent. The swing group has been decisive in three consecutive midterm election waves, backing Republicans by 19 points in 2010 and 12 points in 2014, but supporting Democrats by 18 points in 2006 as they retook control of the House.

Although the president's party nearly always loses seats in Congress in midterm elections, Trump's 36 percent job-approval rating puts Republicans at particular risk this year.

Antipathy for Trump is concentrated among both women and independents, helping to explain Democrats' strong standing. Six in 10 female registered voters "strongly disapprove" of Trump's job performance compared with just over 4 in 10 men. About half of independents also hold intensely negative views of Trump, compared with about one-fifth who "strongly approve" of his efforts.

One obstacle for Democrats — which Republicans hope to solidify amid the shutdown — is the perception that they are solely opposing Trump. Among voters who disapprove of Trump and say Democrats are presenting alternatives to his proposals, 9 in 10 favor Democrats over Republicans in congressional races. That drops to just over 7 in 10 among Trump disapprovers who say Democrats are mainly criticizing the president's proposals without presenting alternatives.

Republicans, who lamented the economy's slow growth under President Barack Obama in recent midterm elections, hope that rising economic optimism will shield their party from major losses this year. The Post-ABC poll finds a 58 percent majority of adults saying that the economy is in excellent or good shape, up seven points since Trump's inauguration, although more give Obama credit for its condition than Trump. Republicans lead Democrats by 11 points among voters who rate the economy positively, while trailing by 48 points among those who see it negatively.

This weekend's women's marches — one year after larger demonstrations following Trump's inauguration — showcased the sustained enthusiasm Democrats will depend on in midterm elections where fewer than half of citizens vote.

Republicans had a clear turnout advantage in midterm elections when Obama was president, but the Post-ABC poll suggests that dynamics have shifted with Trump in office.

Democrats hold a 15-point advantage over Republicans on the generic ballot among voters who say they are "absolutely certain" that they will vote, slightly larger than their 12-point edge among registered voters. And although a November poll showed a much tighter race among voters who reported turning out in the 2014 midterm election, Democrats now hold a nine-point edge with this group, suggesting that they may have less of a handicap with voters in low-turnout contests.

The Post-ABC poll was conducted Jan. 15-18 among a random sample of 1,005 adults reached on cell- and landline phones. The margin of sampling error for overall results is plus or minus 3.5 points, and four points among the sample of 846 registered voters.

Emily Guskin contributed to this report.