The number of Americans who consider themselves to be Republicans jumped nearly two percentage points in December to 34.2%. That’s the largest market share for the Republican brand in nearly two years, since January 2006 (see history from January 2004 to present).

At the same time, the number of Democrats fell to 36.3%. That’s down a point compared to a month ago. During 2007, the number of Democrats has ranged from a low of 35.9% in July to a high of 37.8% in February.

These results are based upon tracking surveys of 15,000 adults per month. The margin of sampling error is less than one percentage point, with a 95% level of confidence. Please keep in mind that figures reported in this article are for all adults, not Likely Voters.

Back in May, the Republicans fell to their lowest level of party identification of the past four years (30.8%). Then, the immigration debate raged in Congress and some Republican legislators helped defeat an unpopular Senate immigration bill. Republicans have gained ground in five of the seven months since then. But, the gains in December—1.7 percentage points—matched the total gains for the previous six months combined.

The December gains for the GOP coincide with increased public confidence in the War on Terror. It’s interesting to note that this did not improve President Bush’s Job Approval ratings while helping the Republican Party overall.

The gap between the parties now shows a 2.1 percentage point advantage for the Democrats. That’s the smallest advantage for the Democrats since January 2006. It represents a dramatic change from the previous five months when the gap favored Democrats by a margin between 4.5 and 4.9 percentage points each and every month.

A year ago at this time, the Democrats had a 6.9 percentage point advantage as they prepared to formally take control of Congress following their victories in Election 2006. It remains to be seen whether the Republican gains can last, but it is startling to note that the Democrats have lost two-thirds of the partisan advantage since taking control of Congress.

Democrats continue to have a significant advantage on ten key issues tracked by Rasmussen Reports. Nancy Pelosi's Party also holds a solid lead on the Generic Congressional Ballot. Pelosi herself is viewed favorably by 38% and unfavorably by 51% of voters nationwide. At her peak, shortly after becoming the first woman Speaker of the House, Pelosi was viewed favorably by 49% of voters.