WASHINGTON  Republicans carry substantial advantages as they move into the final month of the fall campaign, but the resilience of vulnerable Democrats is complicating Republican efforts to lock down enough seats to capture the House and take control of the unsettled electoral battleground.

By now, Republicans had hoped to put away a first layer of Democrats and set their sights on a second tier of incumbents. But the fight for control of Congress is more fluid than it seemed at Labor Day, with Democrats mounting strong resistance in some parts of the country as they try to hold off a potential Republican wave in November.

The chances of a Republican takeover in the House remain far greater than in the Senate, according to a race-by-race analysis by The New York Times. But enough contests remain in flux that both parties head into the final four weeks of the campaign with the ability to change the dynamic before Election Day.

Races typically tighten in the final month as voters on both sides become more engaged, and the political climate is no more favorable for Democrats than it has been all year, with no substantial signs of improvement in the economy or the outlook for unemployment.