The largest Republican super PAC defending the party's majority in the House has canceled advertising buys in two suburban districts, a signal that senior Republicans do not believe the longtime incumbents can win this November.

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This year, he faces a first-time candidate, attorney Jason Crow. A Siena College poll conducted for The New York Times showed Crow leading 51 percent to 40 percent.

The CLF, which has spent more on behalf of Republican candidates than any other group or party organization, had already spent $1.5 million to boost Coffman, a four-term congressman.

The CLF had planned to spend heavily on Bishop's behalf beginning in mid-October.

Party committees and outside groups make advertising reservations early in each election cycle, in order to lock in low rates. Those groups do not have to actually pay for the reservations until just days before the ads air, giving groups leeway to cut ad buys in case their candidate falls flat late in the campaign, or pulls away on their own.

The parties avoid cutting reservations as often as possible. Doing so often signals a death knell for a candidate just weeks before Election Day.