CONWAY, Ark.  They have knocked on 170,000 doors, made 700,000 phone calls, sent 2.7 million pieces of mail and spent almost $6 million on television and radio advertising.

That is how badly labor unions, by their own count, want to defeat Senator Blanche Lincoln, a Democrat they once supported. Even though Arkansas’s labor force is one of the least unionized in the country, labor has thrown huge support behind Mrs. Lincoln’s primary challenger, Lt. Gov. Bill Halter, in a runoff election on Tuesday.

The unions have made the race here the centerpiece of a new effort to hold union-backed candidates accountable for their votes after they are elected.

The push is fueled by a frustration that goes back years but that peaked in the health care debate and in the failure of the Democratic-led Senate to bring far-reaching pro-union legislation to the floor. Even as unions lose members and bargaining power across the country, their campaign against Mrs. Lincoln shows that they are determined to flex their political muscles more than ever before.