WASHINGTON  Senator Christopher J. Dodd, the embattled Connecticut Democrat who was facing an increasingly tough bid for a sixth term in the United States Senate, has decided not to seek re-election this year, Democrats familiar with his plans said Wednesday.

Mr. Dodd, 65, a pivotal figure in the major debates now confronting Congress, is to announce his decision at a news conference Wednesday afternoon in Connecticut.

The decision came hours after another Democratic senator, Byron L. Dorgan of North Dakota, also announced that he would not seek re-election this November. The developments underscored the fragility of the Democrats’ 60-vote Senate majority, which is just enough to block Republican filibusters. Democratic incumbents also face serious challenges in Arkansas, Colorado, Nevada and Pennsylvania among other states.

In this case, Mr. Dodd was already considered one of the most vulnerable Democrats facing re-election this November, and party officials had been privately hoping he would step aside. His move opens the way for the state’s highly popular attorney general, Richard Blumenthal, a Democrat, to run. Democrats and Republicans said he would be a much stronger candidate in what is a Democratic state.