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…

Many Democratic incumbents remain vulnerable, but their positions have stabilized in the last month as they have begun running negative advertisements to raise questions about their Republican challengers and shift the focus of voters away from contentious national issues like health care, bailouts and President Obama’s performance…

Representative John A. Boehner of Ohio, the House Republican leader and the likely speaker if his party wins a majority, signaled that he wanted to step up the intensity. Before lawmakers left town to campaign last week, Mr. Boehner contributed $1 million from his fund-raising vehicles and pressed his colleagues into giving the party $3 million from their own campaigns to help pay for television commercials and make sure Republicans do not come up short because of a lack of resources.