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For the party committee chairman, triage decisions involving incumbents can be extra difficult because they are also personal, often involving longtime friends and colleagues.

“These are hard, nasty, tough decisions,” said former Rep. Thomas M. Davis III, who chaired the NRCC in the 2000 and 2002 election cycles. “The guy you told you’d be there for, all of a sudden you can’t be there for him because you’ve got to save somebody else.”

Mr. Cole agreed, saying that informing colleagues that they have been cut off from party committee support is among the toughest duties he has executed in Washington.

“You’re a little bit like a doctor who is delivering bad news to a patient, saying, ‘I’m sorry, you’ve got six weeks to put your affairs in order,’” he said. “A little empathy is not a bad thing. But at the end of the day, you have to live by the data and the advice that you get.”

Such decisions have left bruised egos and fits of frustration among incumbents — even among those whose campaigns are doing well.

“There is nothing more paranoid than a politician who thinks their life is on the line, whether the objective data suggests it is or not,” Mr. Cole said. “It’s very emotional in that way.”

The parties say they base their triage decisions mostly on objective criteria, such as polling, the fundraising prowess of a candidate, demographics and the opponent’s strength, among others. Yet gut instinct also plays a part.

“These are generally metrics decisions, [but] there is some element of subjectivity — you know, on candidate behavior, coordination between the candidate and the national side, so you get a little bit of that,” he said.

Sometimes money spent during one campaign season will bear fruit in subsequent election cycles, as about a half-dozen candidates in whom the NRCC invested but lost in 2008 won House seats two years later.

The proliferation of outside money into races from political action committees and super PACs also has changed and complicated triage calculations.

“It creates a much more uncertain environment,” Rep. Chris Van Hollen of Maryland, the DCCC chairman in the 2008 and 2010 election cycles.

Media purchases by third-party groups — which by law can’t communicate directly with the parties — have allowed the committees to stretch their budgets. But the practice also has resulted in the DCCC and NRCC losing some control and influence in races.

“We have [outside groups] spending more than the committees spend, because they’re raising money in million-dollar chunks, while we’re raising at considerably smaller portions,” Mr. Cole said.

He added that, even in races a party thinks it has locked up, a super PAC can unexpectedly dump $1 million into a race at the last minute to potentially change its trajectory.

“It’s a whole new universe,” Mr. Cole said.