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On the eve of Election Day, the Republican Party appears likely to keep control of the House of Representative regardless of what happens in the presidential race or the battle for control of the Senate. It's a prospect that would come with major implications.

By controlling the House, the Republicans would have the power to block much of Hillary Clinton's agenda, just as they have President Obama's. It would force Clinton to reach compromises with them on budget bills and other must-pass legislation. And it would allow Republicans to hold investigative hearings on existing controversies (such as Clinton's use of a private e-mail as secretary of state) and initiate new inquiries.

If Trump and Senate Republicans upended expectations and won, they could combine with the Republican House to dramatically overhaul Obamacare, pass large tax cuts and gut regulations the Obama administration passed to regulate Wall Street and limit climate change.

At the House level, Republicans have three key advantages: First, incumbents tend to win elections over challengers, and the GOP currently has effectively a 247-188 majority, so 30 House Republicans would have to lose for Democrats to gain control.

Secondly, Democratic voters, particularly people of color, tend to live in urban areas, giving the GOP a key advantage in suburban and rural districts. And, third, in many states, GOP-controlled state legislatures have drawn districts in ways that heavily favor Republicans.

"Republicans remain overwhelming favorites to hold onto their majority. But there is still plenty of uncertainty about the size of that majority: Democrats could gain anywhere from 5 to 20 seats," says David Wasserman, a House analyst for the non-partisan Cook Political Report.

Other non-partisan analyses, including those by the Rothenberg and Gonzales Political Report, the Washington Post's political science blog Monkey Cage, and the University of Virginia Center for Politics, also predict Democrats will gain seats but fall well short of 30.

Geography

Trump's high unfavorable ratings have been a drag on some Republican U.S. Senate hopefuls, who run in statewide races with similar electorates to the presidential candidates.

But the House landscape is much different. The overwhelming majority of House districts (more than 350 of the 435) are dominated by one of the two parties and completely non-competitive.

And even in liberal-leaning states, such as New York, Democratic voters are often clustered together in a way that is unfavorable to the party winning the House. The non-white population in New York City is about 56 percent, compared to 8 percent in one of the GOP-controlled districts in upstate New York.

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That population imbalance helps explain New York's politics. Of the 17 congressional districts in the New York City metropolitan area, Republicans control just 3. But of the 10 districts outside of New York City, Republicans control 6.

Gerrymandering

Gerrymandering is also a significant factor.

North Carolina is about equally divided between Republicans and Democrats, but has 10 Republican House members and just 3 Democrats. In Pennsylvania, 5 of the 18 House members are Democrats. Redistricting in both states happened under Republican-controlled legislatures.

This fall, none of the districts in North Carolina and only three in Pennsylvania are likely to have close races, according to the Rothenberg and Gonzales Political Report, even as those states will have tight contests at the presidential and U.S. Senate level.

“There was a pretty clear partisan gerrymander in North Carolina,” said Eric McGhee, a research fellow at the non-partisan Public Policy Institute of California who has written extensively about House races.

These are not new problems for Democrats.

In 2012, about 59.2 million people voted for Democratic House candidates, compared to 57.6 million who voted for Republicans. And President Obama won nearly every swing state.

But incumbency, demography and redistricting helped Republicans win 234 seats, compared to the Democrats 201.

Ticket-Splitting Could Help GOP

Republicans in Congress have been telling voters for months that a Republican House is important no matter who is the president. Few Republican House candidates are running on Trump's most controversial proposals: barring some Muslims from entering the country, deporting undocumented immigrants and building a large border wall between the United and Mexico.

Speaker Paul Ryan, by announcing earlier this month that he would not campaign for the real estate mogul, was in effect declaring that the official policy of the House GOP is keeping some distance from Trump.

A number of elite Republicans, such as Mitt Romney and John Kasich, are strongly backing Republican congressional candidates while opposing Trump. House Republicans are hoping some rank and file GOP voters do the same.

"I cannot in good conscience vote for Donald Trump and I would never vote for Hillary Clinton," says Rep. Barbara Comstock, who is running for reelection in a district in Northern Virginia, an area where Trump is unpopular.

This approach worked for Republicans in 1996, when GOP nominee Bob Dole lost the presidential race but Republicans retained control of both houses of Congress.

How The Democrats Could Win

The Democrats won 30 seats in 2006, the GOP gained a whopping 63 in 2010. So huge swings are possible in the chamber, particularly if there is a backlash against the leading figure of one of the two parties. Democrats ran against George W. Bush and the Iraq War in 2006, while the GOP blasted President Obama and the Affordable Care Act four years later.

This year, Democratic House candidates across the country are linking their opponents to Trump.

The Democrats have three paths to victory. GOP turnout could be low, if it becomes more clear that Trump is unlikely to win. House Republicans need traditional GOP voters to cast ballots. (Early voting in Florida, North Carolina and other states suggests that GOP turnout will not have a sizable decline from 2012.)

Alternatively, if Democratic turnout reaches the high level of 2008, this could doom Republicans. Latinos traditionally vote at lower rates than blacks and whites. A big jump in Latino turnout could help Democratic House candidates in states like Arizona, Florida and Nevada.

Finally, Clinton has many more on-the-ground organizers and offices in key states compared to Trump, raising the possibility of a substantial gap in turnout. This advantage should not be overstated. Many of the key House swing districts are in states like California and New York, where Clinton's campaign has spent little money on organizing because those states are not competitive at the presidential level.

But particularly in congressional districts in Florida, Iowa, New Hampshire and Virginia, House Democrats could benefit from Clinton's campaign ensuring that liberal-leaning people vote.