In Arkansas, Democratic Sen. Blanche Lincoln struggles against a flood of labor money. In Nevada, Republican senatorial hopeful Sharron Angle benefits from a conservative combination of the Tea Party Express and Club for Growth.
On the eve of the busiest primary night of the year, Lincoln and Angle are among numerous contenders - incumbents and challengers, Republicans and Democrats - whose fortunes are tied to the actions of outside groups acting independently of candidates or political parties.
Party officials often cringe at such freelancing, campaign consultants say it can overwhelm a candidate's own efforts, and those who are targeted are left venting their frustration.
"People need to ... remind themselves that there are a lot of outside interest groups coming in here spending money to try and tell us who we are and buy our votes," Lincoln said over the weekend as private polls pointed to her likely defeat.
A dozen states hold primaries on Tuesday, picking candidates for Congress and governor for the fall in a campaign season marked by economic woe and political upheaval.
Two senators and two House members have been denied renomination in earlier races - one from each party in each house of Congress. Among Republicans, tea party activists have changed the course of several other races to the consternation of the party's establishment.
In a year of danger for incumbents, Lincoln is the most prominent of those in trouble in the week's races.
Republican Gov. Jim Gibbons in Nevada faces a difficult challenge for renomination after a term marked by a messy public divorce. Among Democrats, California Rep. Jane Harman, a member of the moderates' Blue Dog coalition in Congress, has a liberal challenger in her heavily Democratic district.
Regardless of the outcome in Arkansas, organized labor already claims victory.
By denying the two-term moderate Democrat a majority in the May 18 primary and forcing her into a run-off with Lt. Gov. Bill Halter, "We've been able to demonstrate that labor is unified around this campaign of accountability," said Jon Youngdahl, political director of the Service Employees International Union and a key strategist in organized labor's campaign to dump Lincoln. "It sends an important message to other members of Congress."
Youngdahl said Lincoln has supported free-trade legislation that unions oppose, and opposed an option for a government-run insurance option that labor wanted in this year's health care legislation.
He said she went back on a commitment to support legislation to make it easier to organize workers, then opposed a labor-supported nominee to the National Labor Relations Board.
"There has to be some standard of accountability," added Michael Podhorzer, deputy political director at the AFL-CIO.
Records on file at the Federal Election Commission show organized labor has spent at least $5 million in recent weeks to defeat Lincoln.
The service employees union has invested more than $3 million, a union-funded group named Working America has put in more than $1.3 million, and the American Federation of State, County and Municipal Workers an additional $600,000, according to the records on file.
Lincoln has had significant support from two outside groups, Americans for Common Sense, and the American Hospital Association, which have reported spending less than $700,000 combined.
According to political strategists, the effect of outside money can be to make a campaign uncontrollable for the candidate.
The amount of money spent on television advertising by the candidates in hotly competitive races is "just about always exceeded by these organizations," said Terry Nelson, a Republican with long experience in Republican campaigns.
"This means that candidates often have less influence" over their own campaigns.
Both political parties also run independent political operations,
If Lincoln is eager to attack those who attack her, Halter runs quickly from any suggestion his appeal comes largely from labor's efforts.
"I think it's a diversion from the real issue," he said on CNN on Sunday. "The real issue here in this race is who is going to stand up for middle-class Arkansas families."
In Nevada, Angle's primary path has been unlike any other - cleared not only by conservative organizations, but by a group partial to the Democratic incumbent, Majority Leader Harry Reid.
Records on file at the Federal Election Commission show the Club for Growth and Tea Party Express have each spent nearly $500,000 to help Angle or suppress support for pre-race front-runner Sue Lowden. Businessman Danny Tarkanian is the third major contender in the race.
The Patriot Majority has spent an additional $300,000, much of it from unions, on television ads ridiculing Lowden for having suggested consumers barter chickens for health care. The group is run by a long-ago aide to Reid.
The three groups are allies now, but not for long.
"Harry Reid believes that he can beat Sharron Angle," says Mike Connolly, a spokesman for the Club for Growth. "He should be very careful what he wishes for."