WASHINGTON — Republicans count enough competitive races to challenge Democrats for control of the Senate in next year’s elections. But tea party challenges will make it complicated for them.

Many Republicans worry about crowded primaries in states such as Georgia, Iowa and North Carolina where tea partyers and social conservatives are fighting for the nomination. Some feel tea party victors in similar fights prevented Republicans from winning a Senate majority in 2010 and again in 2012.

Democrats hold an effective 55-45 edge in the Senate now. Republicans need a net gain of six seats to capture control. The numbers favor them. Democrats will be defending 21 of 35 seats to be decided next November. President Barack Obama’s falling popularity and the unpopularity of his health care law also could help GOP candidates.