PRINCETON, NJ -- Gallup's generic ballot for Congress for the week of Sept. 13-19 shows a 46% Democratic and 45% Republican split in registered voters' preferences for the midterm congressional elections. It is the second week out of the last three in which the two parties have been virtually tied.

Gallup's tracking shows a shift from a 49% to 43% Republican advantage in August to a 46% to 45% GOP advantage so far in September. Both of these estimates are based on very large samples, with more than 7,000 interviews conducted in August and more than 5,600 so far in September.

Regardless of these modest shifts in voting intentions, Republicans remain significantly more likely than Democrats to say they are "very" enthusiastic about voting in this year's midterm elections.

The enthusiasm gap this past week was 19 percentage points, with 47% of Republicans very enthusiastic about voting, compared with 28% of Democrats. Republicans have enjoyed at least a 10-point advantage on this measure since Gallup began tracking congressional election preferences in March, including margins of 16 points or higher since August.

Given this continuing enthusiasm gap between Republicans and Democrats, even a tie in registered voters' preferences will almost certainly mean the Republicans will garner the most votes on Election Day.

Bottom Line

Republicans have so far this month lost a little of the unprecedented strength they had among registered voters in August. Voting intentions among registered voters from Sept. 1-19 show a virtual tie between Republicans and Democrats, which is down from the six-point Republican advantage in August. Gallup will begin estimating the voting preferences of the probable 2010 electorate in October, which is highly likely to show a larger Republican advantage than is seen among registered voters.

The pace of midterm congressional election campaigning is beginning to pick up, both at the individual district level, and in terms of national news emphasis. Last week's primary elections, for example, focused news attention on the potential impact of more conservative, Tea Party-backed Republican candidates on the general election. It is possible that further shifts in voter sentiment will be seen in the weeks ahead.

Explore more Gallup data relating to the upcoming congressional midterm elections, including Gallup's complete generic ballot trend since 1950, in our Election 2010 key indicators interactive.