WASHINGTON — The most expensive midterm campaign in American history stumbled into Election Day on Tuesday with voters’ interest at record lows and their divisions deep over what they want their government to do in President Obama’s final two years.

Republicans entered the final hours confident they will gain at least six seats and take control of the Senate, but polls showed several races too close to call. Likely runoffs in Georgia and Louisiana, along with late vote counts in Alaska, Colorado and Iowa, will mean Senate control may remain in doubt beyond Tuesday night.

The same could be true for governors’ races in Colorado, New Hampshire, Georgia and Florida.

The uncertainty about the outcome is a fitting match for the mood of the nation. A slowly but steadily improving economy — with six months of strong growth, gasoline below $3 a gallon for the first time in four years and substantial deficit reduction — has not translated into broader optimism. Voters are more inclined toward blame than credit. Instead, they are seemingly worn down by economic struggles and late waves of panic, chiefly about the threats posed by the Islamic State and the possible spread of Ebola.

Polls show voter interest in the election substantially lower than four years ago. The real intensity has been generated by the deluge of broadcast television ads — more than 1.5 million have aired during this election cycle, according to Kantar Media/CMAG, which tracks political advertising.