WASHINGTON — Republicans entered the final weekend before the midterm elections clearly holding the better hand to control the Senate and poised to add to their House majority. But a decidedly sour electorate and a sizable number of undecided voters added a measure of suspense.

The final drama surrounded the Senate, which has been a Democratic bulwark for President Obama since his party lost its House majority in 2010. Republicans need to gain six seats to seize the Senate, and officials in both parties believe there is a path for them to win at least that many.

Yet the races for a number of seats that will decide the majority remained close, polls showed, prompting Republicans to pour additional money into get-out-the-vote efforts in Alaska, Georgia and Iowa. Democrats were doing the same in Colorado, where they were concerned because groups that tend to favor Republicans voted early in large numbers, and in Iowa.

While an air of mystery hung over no fewer than nine Senate races, the only question surrounding the House was how many seats Republicans would add. If they gain a dozen seats, it will give them an advantage not seen since 1948 and potentially consign the Democrats to minority status until congressional redistricting in the 2020s.